diff --git a/services/filestore/.eslintrc b/services/filestore/.eslintrc
index 42a4b5cace..73103de7f6 100644
--- a/services/filestore/.eslintrc
+++ b/services/filestore/.eslintrc
@@ -23,7 +23,8 @@
   "rules": {
     // Swap the no-unused-expressions rule with a more chai-friendly one
     "no-unused-expressions": 0,
-    "chai-friendly/no-unused-expressions": "error"
+    "chai-friendly/no-unused-expressions": "error",
+    "no-console": "error"
   },
   "overrides": [
     {
diff --git a/services/filestore/app/js/FSPersistorManager.js b/services/filestore/app/js/FSPersistor.js
similarity index 75%
rename from services/filestore/app/js/FSPersistorManager.js
rename to services/filestore/app/js/FSPersistor.js
index 862acb9bcb..973c670efd 100644
--- a/services/filestore/app/js/FSPersistorManager.js
+++ b/services/filestore/app/js/FSPersistor.js
@@ -7,6 +7,7 @@ const { promisify, callbackify } = require('util')
 
 const LocalFileWriter = require('./LocalFileWriter').promises
 const { NotFoundError, ReadError, WriteError } = require('./Errors')
+const PersistorHelper = require('./PersistorHelper')
 
 const pipeline = promisify(Stream.pipeline)
 const fsUnlink = promisify(fs.unlink)
@@ -27,7 +28,7 @@ async function sendFile(location, target, source) {
     const targetStream = fs.createWriteStream(`${location}/${filteredTarget}`)
     await pipeline(sourceStream, targetStream)
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to copy the specified file',
       { location, target, source },
@@ -36,11 +37,22 @@ async function sendFile(location, target, source) {
   }
 }
 
-async function sendStream(location, target, sourceStream) {
+async function sendStream(location, target, sourceStream, sourceMd5) {
   const fsPath = await LocalFileWriter.writeStream(sourceStream)
+  if (!sourceMd5) {
+    sourceMd5 = await _getFileMd5HashForPath(fsPath)
+  }
 
   try {
     await sendFile(location, target, fsPath)
+    const destMd5 = await getFileMd5Hash(location, target)
+    if (sourceMd5 !== destMd5) {
+      await LocalFileWriter.deleteFile(`${location}/${filterName(target)}`)
+      throw new WriteError({
+        message: 'md5 hash mismatch',
+        info: { sourceMd5, destMd5, location, target }
+      })
+    }
   } finally {
     await LocalFileWriter.deleteFile(fsPath)
   }
@@ -53,7 +65,7 @@ async function getFileStream(location, name, opts) {
   try {
     opts.fd = await fsOpen(`${location}/${filteredName}`, 'r')
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to open file for streaming',
       { location, filteredName, opts },
@@ -71,7 +83,7 @@ async function getFileSize(location, filename) {
     const stat = await fsStat(fullPath)
     return stat.size
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to stat file',
       { location, filename },
@@ -80,6 +92,18 @@ async function getFileSize(location, filename) {
   }
 }
 
+async function getFileMd5Hash(location, filename) {
+  const fullPath = path.join(location, filterName(filename))
+  try {
+    return await _getFileMd5HashForPath(fullPath)
+  } catch (err) {
+    throw new ReadError({
+      message: 'unable to get md5 hash from file',
+      info: { location, filename }
+    }).withCause(err)
+  }
+}
+
 async function copyFile(location, fromName, toName) {
   const filteredFromName = filterName(fromName)
   const filteredToName = filterName(toName)
@@ -89,7 +113,7 @@ async function copyFile(location, fromName, toName) {
     const targetStream = fs.createWriteStream(`${location}/${filteredToName}`)
     await pipeline(sourceStream, targetStream)
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to copy file',
       { location, filteredFromName, filteredToName },
@@ -103,12 +127,17 @@ async function deleteFile(location, name) {
   try {
     await fsUnlink(`${location}/${filteredName}`)
   } catch (err) {
-    throw _wrapError(
+    const wrappedError = PersistorHelper.wrapError(
       err,
       'failed to delete file',
       { location, filteredName },
       WriteError
     )
+    if (!(wrappedError instanceof NotFoundError)) {
+      // S3 doesn't give us a 404 when a file wasn't there to be deleted, so we
+      // should be consistent here as well
+      throw wrappedError
+    }
   }
 }
 
@@ -119,7 +148,7 @@ async function deleteDirectory(location, name) {
   try {
     await rmrf(`${location}/${filteredName}`)
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to delete directory',
       { location, filteredName },
@@ -137,7 +166,7 @@ async function checkIfFileExists(location, name) {
     if (err.code === 'ENOENT') {
       return false
     }
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to stat file',
       { location, filteredName },
@@ -167,7 +196,7 @@ async function directorySize(location, name) {
       }
     }
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to get directory size',
       { location, name },
@@ -178,25 +207,12 @@ async function directorySize(location, name) {
   return size
 }
 
-function _wrapError(error, message, params, ErrorType) {
-  if (error.code === 'ENOENT') {
-    return new NotFoundError({
-      message: 'no such file or directory',
-      info: params
-    }).withCause(error)
-  } else {
-    return new ErrorType({
-      message: message,
-      info: params
-    }).withCause(error)
-  }
-}
-
 module.exports = {
   sendFile: callbackify(sendFile),
   sendStream: callbackify(sendStream),
   getFileStream: callbackify(getFileStream),
   getFileSize: callbackify(getFileSize),
+  getFileMd5Hash: callbackify(getFileMd5Hash),
   copyFile: callbackify(copyFile),
   deleteFile: callbackify(deleteFile),
   deleteDirectory: callbackify(deleteDirectory),
@@ -207,6 +223,7 @@ module.exports = {
     sendStream,
     getFileStream,
     getFileSize,
+    getFileMd5Hash,
     copyFile,
     deleteFile,
     deleteDirectory,
@@ -214,3 +231,8 @@ module.exports = {
     directorySize
   }
 }
+
+async function _getFileMd5HashForPath(fullPath) {
+  const stream = fs.createReadStream(fullPath)
+  return PersistorHelper.calculateStreamMd5(stream)
+}
diff --git a/services/filestore/app/js/MigrationPersistor.js b/services/filestore/app/js/MigrationPersistor.js
new file mode 100644
index 0000000000..3ddc762922
--- /dev/null
+++ b/services/filestore/app/js/MigrationPersistor.js
@@ -0,0 +1,228 @@
+const metrics = require('metrics-sharelatex')
+const Settings = require('settings-sharelatex')
+const logger = require('logger-sharelatex')
+const Stream = require('stream')
+const { callbackify, promisify } = require('util')
+const { NotFoundError, WriteError } = require('./Errors')
+
+const pipeline = promisify(Stream.pipeline)
+
+// Persistor that wraps two other persistors. Talks to the 'primary' by default,
+// but will fall back to an older persistor in the case of a not-found error.
+// If `Settings.filestore.fallback.copyOnMiss` is set, this will copy files from the fallback
+// to the primary, in the event that they are missing.
+//
+// It is unlikely that the bucket/location name will be the same on the fallback
+// as the primary. The bucket names should be overridden in `Settings.filestore.fallback.buckets`
+// e.g.
+// Settings.filestore.fallback.buckets = {
+//   myBucketOnS3: 'myBucketOnGCS'
+// }
+
+module.exports = function(primary, fallback) {
+  function _wrapMethodOnBothPersistors(method) {
+    return async function(bucket, key, ...moreArgs) {
+      const fallbackBucket = _getFallbackBucket(bucket)
+
+      await Promise.all([
+        primary.promises[method](bucket, key, ...moreArgs),
+        fallback.promises[method](fallbackBucket, key, ...moreArgs)
+      ])
+    }
+  }
+
+  async function getFileStreamWithFallback(bucket, key, opts) {
+    const shouldCopy =
+      Settings.filestore.fallback.copyOnMiss && !opts.start && !opts.end
+
+    try {
+      return await primary.promises.getFileStream(bucket, key, opts)
+    } catch (err) {
+      if (err instanceof NotFoundError) {
+        const fallbackBucket = _getFallbackBucket(bucket)
+        const fallbackStream = await fallback.promises.getFileStream(
+          fallbackBucket,
+          key,
+          opts
+        )
+        // tee the stream to the client, and as a copy to the primary (if necessary)
+        // start listening on both straight away so that we don't consume bytes
+        // in one place before the other
+        const returnStream = new Stream.PassThrough()
+        pipeline(fallbackStream, returnStream)
+
+        if (shouldCopy) {
+          const copyStream = new Stream.PassThrough()
+          pipeline(fallbackStream, copyStream)
+
+          _copyStreamFromFallbackAndVerify(
+            copyStream,
+            fallbackBucket,
+            bucket,
+            key,
+            key
+          ).catch(() => {
+            // swallow errors, as this runs in the background and will log a warning
+          })
+        }
+        return returnStream
+      }
+      throw err
+    }
+  }
+
+  async function copyFileWithFallback(bucket, sourceKey, destKey) {
+    try {
+      return await primary.promises.copyFile(bucket, sourceKey, destKey)
+    } catch (err) {
+      if (err instanceof NotFoundError) {
+        const fallbackBucket = _getFallbackBucket(bucket)
+        const fallbackStream = await fallback.promises.getFileStream(
+          fallbackBucket,
+          sourceKey,
+          {}
+        )
+
+        const copyStream = new Stream.PassThrough()
+        pipeline(fallbackStream, copyStream)
+
+        if (Settings.filestore.fallback.copyOnMiss) {
+          const missStream = new Stream.PassThrough()
+          pipeline(fallbackStream, missStream)
+
+          // copy from sourceKey -> sourceKey
+          _copyStreamFromFallbackAndVerify(
+            missStream,
+            fallbackBucket,
+            bucket,
+            sourceKey,
+            sourceKey
+          ).then(() => {
+            // swallow errors, as this runs in the background and will log a warning
+          })
+        }
+        // copy from sourceKey -> destKey
+        return _copyStreamFromFallbackAndVerify(
+          copyStream,
+          fallbackBucket,
+          bucket,
+          sourceKey,
+          destKey
+        )
+      }
+      throw err
+    }
+  }
+
+  function _getFallbackBucket(bucket) {
+    return Settings.filestore.fallback.buckets[bucket]
+  }
+
+  function _wrapFallbackMethod(method) {
+    return async function(bucket, key, ...moreArgs) {
+      try {
+        return await primary.promises[method](bucket, key, ...moreArgs)
+      } catch (err) {
+        if (err instanceof NotFoundError) {
+          const fallbackBucket = _getFallbackBucket(bucket)
+          if (Settings.filestore.fallback.copyOnMiss) {
+            const fallbackStream = await fallback.promises.getFileStream(
+              fallbackBucket,
+              key,
+              {}
+            )
+            // run in background
+            _copyStreamFromFallbackAndVerify(
+              fallbackStream,
+              fallbackBucket,
+              bucket,
+              key,
+              key
+            ).catch(err => {
+              logger.warn({ err }, 'failed to copy file from fallback')
+            })
+          }
+          return fallback.promises[method](fallbackBucket, key, ...moreArgs)
+        }
+        throw err
+      }
+    }
+  }
+
+  async function _copyStreamFromFallbackAndVerify(
+    stream,
+    sourceBucket,
+    destBucket,
+    sourceKey,
+    destKey
+  ) {
+    try {
+      let sourceMd5
+      try {
+        sourceMd5 = await fallback.promises.getFileMd5Hash(
+          sourceBucket,
+          sourceKey
+        )
+      } catch (err) {
+        logger.warn(err, 'error getting md5 hash from fallback persistor')
+      }
+
+      await primary.promises.sendStream(destBucket, destKey, stream, sourceMd5)
+    } catch (err) {
+      const error = new WriteError({
+        message: 'unable to copy file to destination persistor',
+        info: {
+          sourceBucket,
+          destBucket,
+          sourceKey,
+          destKey
+        }
+      }).withCause(err)
+      metrics.inc('fallback.copy.failure')
+
+      try {
+        await primary.promises.deleteFile(destBucket, destKey)
+      } catch (err) {
+        error.info.cleanupError = new WriteError({
+          message: 'unable to clean up destination copy artifact',
+          info: {
+            destBucket,
+            destKey
+          }
+        }).withCause(err)
+      }
+
+      logger.warn({ error }, 'failed to copy file from fallback')
+      throw error
+    }
+  }
+
+  return {
+    primaryPersistor: primary,
+    fallbackPersistor: fallback,
+    sendFile: primary.sendFile,
+    sendStream: primary.sendStream,
+    getFileStream: callbackify(getFileStreamWithFallback),
+    getFileMd5Hash: callbackify(_wrapFallbackMethod('getFileMd5Hash')),
+    deleteDirectory: callbackify(
+      _wrapMethodOnBothPersistors('deleteDirectory')
+    ),
+    getFileSize: callbackify(_wrapFallbackMethod('getFileSize')),
+    deleteFile: callbackify(_wrapMethodOnBothPersistors('deleteFile')),
+    copyFile: callbackify(copyFileWithFallback),
+    checkIfFileExists: callbackify(_wrapFallbackMethod('checkIfFileExists')),
+    directorySize: callbackify(_wrapFallbackMethod('directorySize')),
+    promises: {
+      sendFile: primary.promises.sendFile,
+      sendStream: primary.promises.sendStream,
+      getFileStream: getFileStreamWithFallback,
+      getFileMd5Hash: _wrapFallbackMethod('getFileMd5Hash'),
+      deleteDirectory: _wrapMethodOnBothPersistors('deleteDirectory'),
+      getFileSize: _wrapFallbackMethod('getFileSize'),
+      deleteFile: _wrapMethodOnBothPersistors('deleteFile'),
+      copyFile: copyFileWithFallback,
+      checkIfFileExists: _wrapFallbackMethod('checkIfFileExists'),
+      directorySize: _wrapFallbackMethod('directorySize')
+    }
+  }
+}
diff --git a/services/filestore/app/js/PersistorHelper.js b/services/filestore/app/js/PersistorHelper.js
new file mode 100644
index 0000000000..ea8132a9c9
--- /dev/null
+++ b/services/filestore/app/js/PersistorHelper.js
@@ -0,0 +1,105 @@
+const crypto = require('crypto')
+const meter = require('stream-meter')
+const Stream = require('stream')
+const logger = require('logger-sharelatex')
+const { WriteError, ReadError, NotFoundError } = require('./Errors')
+const { promisify } = require('util')
+
+const pipeline = promisify(Stream.pipeline)
+
+module.exports = {
+  calculateStreamMd5,
+  verifyMd5,
+  getMeteredStream,
+  waitForStreamReady,
+  wrapError
+}
+
+// returns a promise which resolves with the md5 hash of the stream
+function calculateStreamMd5(stream) {
+  const hash = crypto.createHash('md5')
+  hash.setEncoding('hex')
+
+  return pipeline(stream, hash).then(() => hash.read())
+}
+
+// verifies the md5 hash of a file against the supplied md5 or the one stored in
+// storage if not supplied - deletes the new file if the md5 does not match and
+// throws an error
+async function verifyMd5(persistor, bucket, key, sourceMd5, destMd5 = null) {
+  if (!destMd5) {
+    destMd5 = await persistor.promises.getFileMd5Hash(bucket, key)
+  }
+
+  if (sourceMd5 !== destMd5) {
+    try {
+      await persistor.promises.deleteFile(bucket, key)
+    } catch (err) {
+      logger.warn(err, 'error deleting file for invalid upload')
+    }
+
+    throw new WriteError({
+      message: 'source and destination hashes do not match',
+      info: {
+        sourceMd5,
+        destMd5,
+        bucket,
+        key
+      }
+    })
+  }
+}
+
+// returns the next stream in the pipeline, and calls the callback with the byte count
+// when the stream finishes or receives an error
+function getMeteredStream(stream, callback) {
+  const meteredStream = meter()
+
+  pipeline(stream, meteredStream)
+    .then(() => {
+      callback(null, meteredStream.bytes)
+    })
+    .catch(err => {
+      // on error, just send how many bytes we received before the stream stopped
+      callback(err, meteredStream.bytes)
+    })
+
+  return meteredStream
+}
+
+// resolves when a stream is 'readable', or rejects if the stream throws an error
+// before that happens - this lets us handle protocol-level errors before trying
+// to read them
+function waitForStreamReady(stream) {
+  return new Promise((resolve, reject) => {
+    const onError = function(err) {
+      reject(wrapError(err, 'error before stream became ready', {}, ReadError))
+    }
+    const onStreamReady = function() {
+      stream.removeListener('readable', onStreamReady)
+      stream.removeListener('error', onError)
+      resolve(stream)
+    }
+    stream.on('readable', onStreamReady)
+    stream.on('error', onError)
+  })
+}
+
+function wrapError(error, message, params, ErrorType) {
+  if (
+    error instanceof NotFoundError ||
+    ['NoSuchKey', 'NotFound', 404, 'AccessDenied', 'ENOENT'].includes(
+      error.code
+    )
+  ) {
+    return new NotFoundError({
+      message: 'no such file',
+      info: params
+    }).withCause(error)
+  } else {
+    return new ErrorType({
+      message: message,
+      info: params
+    }).withCause(error)
+  }
+}
diff --git a/services/filestore/app/js/PersistorManager.js b/services/filestore/app/js/PersistorManager.js
index cca0cf0f36..32f6cd41f8 100644
--- a/services/filestore/app/js/PersistorManager.js
+++ b/services/filestore/app/js/PersistorManager.js
@@ -3,7 +3,8 @@ const logger = require('logger-sharelatex')
 
 logger.log(
   {
-    backend: settings.filestore.backend
+    backend: settings.filestore.backend,
+    fallback: settings.filestore.fallback && settings.filestore.fallback.backend
   },
   'Loading backend'
 )
@@ -11,14 +12,26 @@ if (!settings.filestore.backend) {
   throw new Error('no backend specified - config incomplete')
 }
 
-switch (settings.filestore.backend) {
-  case 'aws-sdk':
-  case 's3':
-    module.exports = require('./S3PersistorManager')
-    break
-  case 'fs':
-    module.exports = require('./FSPersistorManager')
-    break
-  default:
-    throw new Error(`unknown filestore backend: ${settings.filestore.backend}`)
+function getPersistor(backend) {
+  switch (backend) {
+    case 'aws-sdk':
+    case 's3':
+      return require('./S3Persistor')
+    case 'fs':
+      return require('./FSPersistor')
+    default:
+      throw new Error(`unknown filestore backend: ${backend}`)
+  }
 }
+
+let persistor = getPersistor(settings.filestore.backend)
+
+if (settings.filestore.fallback && settings.filestore.fallback.backend) {
+  const migrationPersistor = require('./MigrationPersistor')
+  persistor = migrationPersistor(
+    persistor,
+    getPersistor(settings.filestore.fallback.backend)
+  )
+}
+
+module.exports = persistor
diff --git a/services/filestore/app/js/S3PersistorManager.js b/services/filestore/app/js/S3Persistor.js
similarity index 63%
rename from services/filestore/app/js/S3PersistorManager.js
rename to services/filestore/app/js/S3Persistor.js
index 52cadfbfbd..891d7be68e 100644
--- a/services/filestore/app/js/S3PersistorManager.js
+++ b/services/filestore/app/js/S3Persistor.js
@@ -6,7 +6,8 @@ https.globalAgent.maxSockets = 300
 const settings = require('settings-sharelatex')
 const metrics = require('metrics-sharelatex')
 
-const meter = require('stream-meter')
+const PersistorHelper = require('./PersistorHelper')
+
 const fs = require('fs')
 const S3 = require('aws-sdk/clients/s3')
 const { URL } = require('url')
@@ -18,10 +19,11 @@ const {
   SettingsError
 } = require('./Errors')
 
-module.exports = {
+const S3Persistor = {
   sendFile: callbackify(sendFile),
   sendStream: callbackify(sendStream),
   getFileStream: callbackify(getFileStream),
+  getFileMd5Hash: callbackify(getFileMd5Hash),
   deleteDirectory: callbackify(deleteDirectory),
   getFileSize: callbackify(getFileSize),
   deleteFile: callbackify(deleteFile),
@@ -32,6 +34,7 @@ module.exports = {
     sendFile,
     sendStream,
     getFileStream,
+    getFileMd5Hash,
     deleteDirectory,
     getFileSize,
     deleteFile,
@@ -41,12 +44,18 @@ module.exports = {
   }
 }
 
+module.exports = S3Persistor
+
+function hexToBase64(hex) {
+  return Buffer.from(hex, 'hex').toString('base64')
+}
+
 async function sendFile(bucketName, key, fsPath) {
   let readStream
   try {
     readStream = fs.createReadStream(fsPath)
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'error reading file from disk',
       { bucketName, key, fsPath },
@@ -56,22 +65,56 @@ async function sendFile(bucketName, key, fsPath) {
   return sendStream(bucketName, key, readStream)
 }
 
-async function sendStream(bucketName, key, readStream) {
+async function sendStream(bucketName, key, readStream, sourceMd5) {
   try {
-    const meteredStream = meter()
-    meteredStream.on('finish', () => {
-      metrics.count('s3.egress', meteredStream.bytes)
-    })
+    // if there is no supplied md5 hash, we calculate the hash as the data passes through
+    let hashPromise
+    let b64Hash
 
-    await _getClientForBucket(bucketName)
-      .upload({
-        Bucket: bucketName,
-        Key: key,
-        Body: readStream.pipe(meteredStream)
-      })
+    if (sourceMd5) {
+      b64Hash = hexToBase64(sourceMd5)
+    } else {
+      hashPromise = PersistorHelper.calculateStreamMd5(readStream)
+    }
+
+    const meteredStream = PersistorHelper.getMeteredStream(
+      readStream,
+      (_, byteCount) => {
+        // ignore the error parameter and just log the byte count
+        metrics.count('s3.egress', byteCount)
+      }
+    )
+
+    // if we have an md5 hash, pass this to S3 to verify the upload
+    const uploadOptions = {
+      Bucket: bucketName,
+      Key: key,
+      Body: meteredStream
+    }
+    if (b64Hash) {
+      uploadOptions.ContentMD5 = b64Hash
+    }
+
+    const response = await _getClientForBucket(bucketName)
+      .upload(uploadOptions)
       .promise()
+    const destMd5 = _md5FromResponse(response)
+
+    // if we didn't have an md5 hash, we should compare our computed one with S3's
+    // as we couldn't tell S3 about it beforehand
+    if (hashPromise) {
+      sourceMd5 = await hashPromise
+      // throws on mismatch
+      await PersistorHelper.verifyMd5(
+        S3Persistor,
+        bucketName,
+        key,
+        sourceMd5,
+        destMd5
+      )
+    }
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'upload to S3 failed',
       { bucketName, key },
@@ -91,25 +134,29 @@ async function getFileStream(bucketName, key, opts) {
     params.Range = `bytes=${opts.start}-${opts.end}`
   }
 
-  return new Promise((resolve, reject) => {
-    const stream = _getClientForBucket(bucketName)
-      .getObject(params)
-      .createReadStream()
+  const stream = _getClientForBucket(bucketName)
+    .getObject(params)
+    .createReadStream()
 
-    const meteredStream = meter()
-    meteredStream.on('finish', () => {
-      metrics.count('s3.ingress', meteredStream.bytes)
-    })
-
-    const onStreamReady = function() {
-      stream.removeListener('readable', onStreamReady)
-      resolve(stream.pipe(meteredStream))
+  const meteredStream = PersistorHelper.getMeteredStream(
+    stream,
+    (_, byteCount) => {
+      // ignore the error parameter and just log the byte count
+      metrics.count('s3.ingress', byteCount)
     }
-    stream.on('readable', onStreamReady)
-    stream.on('error', err => {
-      reject(_wrapError(err, 'error reading from S3', params, ReadError))
-    })
-  })
+  )
+
+  try {
+    await PersistorHelper.waitForStreamReady(stream)
+    return meteredStream
+  } catch (err) {
+    throw PersistorHelper.wrapError(
+      err,
+      'error reading file from S3',
+      { bucketName, key, opts },
+      ReadError
+    )
+  }
 }
 
 async function deleteDirectory(bucketName, key) {
@@ -120,7 +167,7 @@ async function deleteDirectory(bucketName, key) {
       .listObjects({ Bucket: bucketName, Prefix: key })
       .promise()
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'failed to list objects in S3',
       { bucketName, key },
@@ -141,7 +188,7 @@ async function deleteDirectory(bucketName, key) {
         })
         .promise()
     } catch (err) {
-      throw _wrapError(
+      throw PersistorHelper.wrapError(
         err,
         'failed to delete objects in S3',
         { bucketName, key },
@@ -158,7 +205,7 @@ async function getFileSize(bucketName, key) {
       .promise()
     return response.ContentLength
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'error getting size of s3 object',
       { bucketName, key },
@@ -167,13 +214,31 @@ async function getFileSize(bucketName, key) {
   }
 }
 
+async function getFileMd5Hash(bucketName, key) {
+  try {
+    const response = await _getClientForBucket(bucketName)
+      .headObject({ Bucket: bucketName, Key: key })
+      .promise()
+    const md5 = _md5FromResponse(response)
+    return md5
+  } catch (err) {
+    throw PersistorHelper.wrapError(
+      err,
+      'error getting hash of s3 object',
+      { bucketName, key },
+      ReadError
+    )
+  }
+}
+
 async function deleteFile(bucketName, key) {
   try {
     await _getClientForBucket(bucketName)
       .deleteObject({ Bucket: bucketName, Key: key })
       .promise()
   } catch (err) {
-    throw _wrapError(
+    // s3 does not give us a NotFoundError here
+    throw PersistorHelper.wrapError(
       err,
       'failed to delete file in S3',
       { bucketName, key },
@@ -193,7 +258,12 @@ async function copyFile(bucketName, sourceKey, destKey) {
       .copyObject(params)
       .promise()
   } catch (err) {
-    throw _wrapError(err, 'failed to copy file in S3', params, WriteError)
+    throw PersistorHelper.wrapError(
+      err,
+      'failed to copy file in S3',
+      params,
+      WriteError
+    )
   }
 }
 
@@ -205,7 +275,7 @@ async function checkIfFileExists(bucketName, key) {
     if (err instanceof NotFoundError) {
       return false
     }
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'error checking whether S3 object exists',
       { bucketName, key },
@@ -222,7 +292,7 @@ async function directorySize(bucketName, key) {
 
     return response.Contents.reduce((acc, item) => item.Size + acc, 0)
   } catch (err) {
-    throw _wrapError(
+    throw PersistorHelper.wrapError(
       err,
       'error getting directory size in S3',
       { bucketName, key },
@@ -231,22 +301,6 @@ async function directorySize(bucketName, key) {
   }
 }
 
-function _wrapError(error, message, params, ErrorType) {
-  if (
-    ['NoSuchKey', 'NotFound', 'AccessDenied', 'ENOENT'].includes(error.code)
-  ) {
-    return new NotFoundError({
-      message: 'no such file',
-      info: params
-    }).withCause(error)
-  } else {
-    return new ErrorType({
-      message: message,
-      info: params
-    }).withCause(error)
-  }
-}
-
 const _clients = new Map()
 let _defaultClient
 
@@ -309,3 +363,18 @@ function _buildClientOptions(bucketCredentials) {
 
   return options
 }
+
+function _md5FromResponse(response) {
+  const md5 = (response.ETag || '').replace(/[ "]/g, '')
+  if (!md5.match(/^[a-f0-9]{32}$/)) {
+    throw new ReadError({
+      message: 's3 etag not in md5-hash format',
+      info: {
+        md5,
+        eTag: response.ETag
+      }
+    })
+  }
+
+  return md5
+}
diff --git a/services/filestore/config/settings.defaults.coffee b/services/filestore/config/settings.defaults.coffee
index 206f932a76..bb124ae8e0 100644
--- a/services/filestore/config/settings.defaults.coffee
+++ b/services/filestore/config/settings.defaults.coffee
@@ -7,6 +7,19 @@ if process.env['AWS_KEY'] && !process.env['AWS_ACCESS_KEY_ID']
 if process.env['AWS_SECRET'] && !process.env['AWS_SECRET_ACCESS_KEY']
 	process.env['AWS_SECRET_ACCESS_KEY'] = process.env['AWS_SECRET']
 
+# pre-backend setting, fall back to old behaviour
+unless process.env['BACKEND']?
+	if process.env['AWS_ACCESS_KEY_ID']? or process.env['S3_BUCKET_CREDENTIALS']?
+		process.env['BACKEND'] = "s3"
+		process.env['USER_FILES_BUCKET_NAME'] = process.env['AWS_S3_USER_FILES_BUCKET_NAME']
+		process.env['TEMPLATE_FILES_BUCKET_NAME'] = process.env['AWS_S3_TEMPLATE_FILES_BUCKET_NAME']
+		process.env['PUBLIC_FILES_BUCKET_NAME'] = process.env['AWS_S3_PUBLIC_FILES_BUCKET_NAME']
+	else
+		process.env['BACKEND'] = "fs"
+		process.env['USER_FILES_BUCKET_NAME'] = Path.resolve(__dirname + "/../user_files")
+		process.env['TEMPLATE_FILES_BUCKET_NAME'] = Path.resolve(__dirname + "/../template_files")
+		process.env['PUBLIC_FILES_BUCKET_NAME'] = Path.resolve(__dirname + "/../public_files")
+
 settings =
 	internal:
 		filestore:
@@ -18,38 +31,28 @@ settings =
 		# Choices are
 		# s3 - Amazon S3
 		# fs - local filesystem
-		if process.env['AWS_ACCESS_KEY_ID']? or process.env['S3_BUCKET_CREDENTIALS']?
-			backend: "s3"
-			s3:
+		backend: process.env['BACKEND']
+
+		s3:
+			if process.env['AWS_ACCESS_KEY_ID']? or process.env['S3_BUCKET_CREDENTIALS']?
 				key: process.env['AWS_ACCESS_KEY_ID']
 				secret: process.env['AWS_SECRET_ACCESS_KEY']
 				endpoint: process.env['AWS_S3_ENDPOINT']
-			stores:
-				user_files: process.env['AWS_S3_USER_FILES_BUCKET_NAME']
-				template_files: process.env['AWS_S3_TEMPLATE_FILES_BUCKET_NAME']
-				public_files: process.env['AWS_S3_PUBLIC_FILES_BUCKET_NAME']
-			# if you are using S3, then fill in your S3 details below,
-			# or use env var with the same structure.
-			# s3:
-			# 	key: ""     # default
-			# 	secret: ""  # default
-			#
-			# s3BucketCreds:
-			#   bucketname1: # secrets for bucketname1
-			#     auth_key: ""
-			#     auth_secret: ""
-			#  bucketname2: # secrets for bucketname2...
-			s3BucketCreds: JSON.parse process.env['S3_BUCKET_CREDENTIALS'] if process.env['S3_BUCKET_CREDENTIALS']?
-		else
-			backend: "fs"
-			stores:
-				#
-				# For local filesystem this is the directory to store the files in.
-				# Must contain full path, e.g. "/var/lib/sharelatex/data".
-				# This path must exist, not be tmpfs and be writable to by the user sharelatex is run as.
-				user_files: Path.resolve(__dirname + "/../user_files")
-				public_files: Path.resolve(__dirname + "/../public_files")
-				template_files: Path.resolve(__dirname + "/../template_files")
+
+		stores:
+			user_files: process.env['USER_FILES_BUCKET_NAME']
+			template_files: process.env['TEMPLATE_FILES_BUCKET_NAME']
+			public_files: process.env['PUBLIC_FILES_BUCKET_NAME']
+
+		s3BucketCreds: JSON.parse process.env['S3_BUCKET_CREDENTIALS'] if process.env['S3_BUCKET_CREDENTIALS']?
+
+		fallback:
+			if process.env['FALLBACK_BACKEND']?
+				backend: process.env['FALLBACK_BACKEND']
+				# mapping of bucket names on the fallback, to bucket names on the primary.
+				# e.g. { myS3UserFilesBucketName: 'myGoogleUserFilesBucketName' }
+				buckets: JSON.parse(process.env['FALLBACK_BUCKET_MAPPING'] || '{}')
+				copyOnMiss: process.env['COPY_ON_MISS'] == 'true'
 
 	path:
 		uploadFolder: Path.resolve(__dirname + "/../uploads")
diff --git a/services/filestore/npm-shrinkwrap.json b/services/filestore/npm-shrinkwrap.json
index 8d78271caa..bdc836e237 100644
--- a/services/filestore/npm-shrinkwrap.json
+++ b/services/filestore/npm-shrinkwrap.json
@@ -606,11 +606,6 @@
         "event-target-shim": "^5.0.0"
       }
     },
-    "accept-encoding": {
-      "version": "0.1.0",
-      "resolved": "https://registry.npmjs.org/accept-encoding/-/accept-encoding-0.1.0.tgz",
-      "integrity": "sha1-XdiLjfcfHcLlzGuVZezOHjmaMz4="
-    },
     "accepts": {
       "version": "1.3.5",
       "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.5.tgz",
@@ -790,11 +785,6 @@
         }
       }
     },
-    "aws-sign": {
-      "version": "0.2.1",
-      "resolved": "https://registry.npmjs.org/aws-sign/-/aws-sign-0.2.1.tgz",
-      "integrity": "sha1-uWGyLwuqTxXsJBFA83dtbBQoVtA="
-    },
     "aws-sign2": {
       "version": "0.7.0",
       "resolved": "https://registry.npmjs.org/aws-sign2/-/aws-sign2-0.7.0.tgz",
@@ -857,14 +847,6 @@
         "tweetnacl": "^0.14.3"
       }
     },
-    "best-encoding": {
-      "version": "0.1.1",
-      "resolved": "https://registry.npmjs.org/best-encoding/-/best-encoding-0.1.1.tgz",
-      "integrity": "sha1-GVIT2rysBFgYuAe3ox+Dn63cl04=",
-      "requires": {
-        "accept-encoding": "~0.1.0"
-      }
-    },
     "bignumber.js": {
       "version": "7.2.1",
       "resolved": "https://registry.npmjs.org/bignumber.js/-/bignumber.js-7.2.1.tgz",
@@ -883,14 +865,6 @@
       "resolved": "https://registry.npmjs.org/bintrees/-/bintrees-1.0.1.tgz",
       "integrity": "sha1-DmVcm5wkNeqraL9AJyJtK1WjRSQ="
     },
-    "bl": {
-      "version": "0.7.0",
-      "resolved": "https://registry.npmjs.org/bl/-/bl-0.7.0.tgz",
-      "integrity": "sha1-P7BnBgKsKHjrdw3CA58YNr5irls=",
-      "requires": {
-        "readable-stream": "~1.0.2"
-      }
-    },
     "body-parser": {
       "version": "1.18.3",
       "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-1.18.3.tgz",
@@ -914,14 +888,6 @@
       "integrity": "sha1-tcCeF8rNET0Rt7s+04TMASmU2Gs=",
       "dev": true
     },
-    "boom": {
-      "version": "0.3.8",
-      "resolved": "https://registry.npmjs.org/boom/-/boom-0.3.8.tgz",
-      "integrity": "sha1-yM2wQUNZEnQWKMBE7Mcy0dF8Ceo=",
-      "requires": {
-        "hoek": "0.7.x"
-      }
-    },
     "brace-expansion": {
       "version": "1.1.11",
       "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-1.1.11.tgz",
@@ -995,11 +961,6 @@
         "quick-lru": "^4.0.1"
       }
     },
-    "caseless": {
-      "version": "0.3.0",
-      "resolved": "https://registry.npmjs.org/caseless/-/caseless-0.3.0.tgz",
-      "integrity": "sha1-U06XkWOH07cGtk/eu6xGQ4RQk08="
-    },
     "chai": {
       "version": "4.2.0",
       "resolved": "https://registry.npmjs.org/chai/-/chai-4.2.0.tgz",
@@ -1117,14 +1078,6 @@
       "integrity": "sha1-p9BVi9icQveV3UIyj3QIMcpTvCU=",
       "dev": true
     },
-    "combined-stream": {
-      "version": "0.0.7",
-      "resolved": "https://registry.npmjs.org/combined-stream/-/combined-stream-0.0.7.tgz",
-      "integrity": "sha1-ATfmV7qlp1QcV6w3rF/AfXO03B8=",
-      "requires": {
-        "delayed-stream": "0.0.5"
-      }
-    },
     "common-tags": {
       "version": "1.8.0",
       "resolved": "https://registry.npmjs.org/common-tags/-/common-tags-1.8.0.tgz",
@@ -1171,11 +1124,6 @@
       "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.3.1.tgz",
       "integrity": "sha1-5+Ch+e9DtMi6klxcWpboBtFoc7s="
     },
-    "cookie-jar": {
-      "version": "0.2.0",
-      "resolved": "https://registry.npmjs.org/cookie-jar/-/cookie-jar-0.2.0.tgz",
-      "integrity": "sha1-ZOzAasl423leS1KQy+SLo3gUAPo="
-    },
     "cookie-signature": {
       "version": "1.0.6",
       "resolved": "https://registry.npmjs.org/cookie-signature/-/cookie-signature-1.0.6.tgz",
@@ -1213,14 +1161,6 @@
         }
       }
     },
-    "cryptiles": {
-      "version": "0.1.3",
-      "resolved": "https://registry.npmjs.org/cryptiles/-/cryptiles-0.1.3.tgz",
-      "integrity": "sha1-GlVnNPBtJLo0hirpy55wmjr7/xw=",
-      "requires": {
-        "boom": "0.3.x"
-      }
-    },
     "dashdash": {
       "version": "1.14.1",
       "resolved": "https://registry.npmjs.org/dashdash/-/dashdash-1.14.1.tgz",
@@ -1272,11 +1212,6 @@
       "resolved": "https://registry.npmjs.org/delay/-/delay-4.3.0.tgz",
       "integrity": "sha1-7+6/uPVFV5yzlrOnIkQ+yW0UxQ4="
     },
-    "delayed-stream": {
-      "version": "0.0.5",
-      "resolved": "https://registry.npmjs.org/delayed-stream/-/delayed-stream-0.0.5.tgz",
-      "integrity": "sha1-1LH0OpPoKW3+AmlPRoC8N6MTxz8="
-    },
     "depd": {
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/depd/-/depd-1.1.2.tgz",
@@ -2078,28 +2013,6 @@
         }
       }
     },
-    "forever-agent": {
-      "version": "0.2.0",
-      "resolved": "https://registry.npmjs.org/forever-agent/-/forever-agent-0.2.0.tgz",
-      "integrity": "sha1-4cJcetROCcOPIzh2x2/MJP+EOx8="
-    },
-    "form-data": {
-      "version": "0.0.10",
-      "resolved": "https://registry.npmjs.org/form-data/-/form-data-0.0.10.tgz",
-      "integrity": "sha1-2zRaU3jYau6x7V1VO4aawZLS9e0=",
-      "requires": {
-        "async": "~0.2.7",
-        "combined-stream": "~0.0.4",
-        "mime": "~1.2.2"
-      },
-      "dependencies": {
-        "mime": {
-          "version": "1.2.11",
-          "resolved": "https://registry.npmjs.org/mime/-/mime-1.2.11.tgz",
-          "integrity": "sha1-WCA+7Ybjpe8XrtK32evUfwpg3RA="
-        }
-      }
-    },
     "forwarded": {
       "version": "0.1.2",
       "resolved": "https://registry.npmjs.org/forwarded/-/forwarded-0.1.2.tgz",
@@ -2323,17 +2236,6 @@
       "integrity": "sha512-PLcsoqu++dmEIZB+6totNFKq/7Do+Z0u4oT0zKOJNl3lYK6vGwwu2hjHs+68OEZbTjiUE9bgOABXbP/GvrS0Kg==",
       "dev": true
     },
-    "hawk": {
-      "version": "0.10.2",
-      "resolved": "https://registry.npmjs.org/hawk/-/hawk-0.10.2.tgz",
-      "integrity": "sha1-mzYd7pWpMWQObVBOBWCaj8OsRdI=",
-      "requires": {
-        "boom": "0.3.x",
-        "cryptiles": "0.1.x",
-        "hoek": "0.7.x",
-        "sntp": "0.1.x"
-      }
-    },
     "he": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/he/-/he-1.1.1.tgz",
@@ -2349,11 +2251,6 @@
       "resolved": "https://registry.npmjs.org/hex2dec/-/hex2dec-1.1.2.tgz",
       "integrity": "sha1-jhzkvvNqdPfVcjw/swkMKGAHczg="
     },
-    "hoek": {
-      "version": "0.7.6",
-      "resolved": "https://registry.npmjs.org/hoek/-/hoek-0.7.6.tgz",
-      "integrity": "sha1-YPvZBFV1Qc0rh5Wr8wihs3cOFVo="
-    },
     "hosted-git-info": {
       "version": "2.8.5",
       "resolved": "https://registry.npmjs.org/hosted-git-info/-/hosted-git-info-2.8.5.tgz",
@@ -2701,28 +2598,6 @@
         "graceful-fs": "^4.1.9"
       }
     },
-    "knox": {
-      "version": "0.9.2",
-      "resolved": "https://registry.npmjs.org/knox/-/knox-0.9.2.tgz",
-      "integrity": "sha1-NzZZNmniTwJP2vcjtqHcSv2DmnE=",
-      "requires": {
-        "debug": "^1.0.2",
-        "mime": "*",
-        "once": "^1.3.0",
-        "stream-counter": "^1.0.0",
-        "xml2js": "^0.4.4"
-      },
-      "dependencies": {
-        "debug": {
-          "version": "1.0.5",
-          "resolved": "https://registry.npmjs.org/debug/-/debug-1.0.5.tgz",
-          "integrity": "sha1-9yQSF0MPmd7EwrRz6rkiKOh0wqw=",
-          "requires": {
-            "ms": "2.0.0"
-          }
-        }
-      }
-    },
     "levn": {
       "version": "0.3.0",
       "resolved": "https://registry.npmjs.org/levn/-/levn-0.3.0.tgz",
@@ -3353,55 +3228,6 @@
       "resolved": "https://registry.npmjs.org/node-forge/-/node-forge-0.8.4.tgz",
       "integrity": "sha1-1nOGYrZhvhnicR7wGqOxghLxMDA="
     },
-    "node-transloadit": {
-      "version": "0.0.4",
-      "resolved": "https://registry.npmjs.org/node-transloadit/-/node-transloadit-0.0.4.tgz",
-      "integrity": "sha1-4ZoHheON94NblO2AANHjXmg7zsE=",
-      "requires": {
-        "request": "~2.16.6",
-        "underscore": "1.2.1"
-      },
-      "dependencies": {
-        "json-stringify-safe": {
-          "version": "3.0.0",
-          "resolved": "https://registry.npmjs.org/json-stringify-safe/-/json-stringify-safe-3.0.0.tgz",
-          "integrity": "sha1-nbew5TDH8onF6MhDKvGRwv91pbM="
-        },
-        "mime": {
-          "version": "1.2.11",
-          "resolved": "https://registry.npmjs.org/mime/-/mime-1.2.11.tgz",
-          "integrity": "sha1-WCA+7Ybjpe8XrtK32evUfwpg3RA="
-        },
-        "qs": {
-          "version": "0.5.6",
-          "resolved": "https://registry.npmjs.org/qs/-/qs-0.5.6.tgz",
-          "integrity": "sha1-MbGtBYVnZRxSaSFQa5qHk5EaA4Q="
-        },
-        "request": {
-          "version": "2.16.6",
-          "resolved": "https://registry.npmjs.org/request/-/request-2.16.6.tgz",
-          "integrity": "sha1-hy/kRa5y3iZrN4edatfclI+gHK0=",
-          "requires": {
-            "aws-sign": "~0.2.0",
-            "cookie-jar": "~0.2.0",
-            "forever-agent": "~0.2.0",
-            "form-data": "~0.0.3",
-            "hawk": "~0.10.2",
-            "json-stringify-safe": "~3.0.0",
-            "mime": "~1.2.7",
-            "node-uuid": "~1.4.0",
-            "oauth-sign": "~0.2.0",
-            "qs": "~0.5.4",
-            "tunnel-agent": "~0.2.0"
-          }
-        },
-        "underscore": {
-          "version": "1.2.1",
-          "resolved": "https://registry.npmjs.org/underscore/-/underscore-1.2.1.tgz",
-          "integrity": "sha1-/FxrB2VnPZKi1KyLTcCqiHAuK9Q="
-        }
-      }
-    },
     "node-uuid": {
       "version": "1.4.8",
       "resolved": "https://registry.npmjs.org/node-uuid/-/node-uuid-1.4.8.tgz",
@@ -3427,11 +3253,6 @@
         }
       }
     },
-    "oauth-sign": {
-      "version": "0.2.0",
-      "resolved": "https://registry.npmjs.org/oauth-sign/-/oauth-sign-0.2.0.tgz",
-      "integrity": "sha1-oOahcV2u0GLzIrYit/5a/RA1tuI="
-    },
     "object-inspect": {
       "version": "1.7.0",
       "resolved": "https://registry.npmjs.org/object-inspect/-/object-inspect-1.7.0.tgz",
@@ -4446,29 +4267,6 @@
         "read-pkg": "^2.0.0"
       }
     },
-    "readable-stream": {
-      "version": "1.0.34",
-      "resolved": "https://registry.npmjs.org/readable-stream/-/readable-stream-1.0.34.tgz",
-      "integrity": "sha1-Elgg40vIQtLyqq+v5MKRbuMsFXw=",
-      "requires": {
-        "core-util-is": "~1.0.0",
-        "inherits": "~2.0.1",
-        "isarray": "0.0.1",
-        "string_decoder": "~0.10.x"
-      },
-      "dependencies": {
-        "isarray": {
-          "version": "0.0.1",
-          "resolved": "https://registry.npmjs.org/isarray/-/isarray-0.0.1.tgz",
-          "integrity": "sha1-ihis/Kmo9Bd+Cav8YDiTmwXR7t8="
-        }
-      }
-    },
-    "recluster": {
-      "version": "0.3.7",
-      "resolved": "https://registry.npmjs.org/recluster/-/recluster-0.3.7.tgz",
-      "integrity": "sha1-aKRx3ZC2obl3ZjTPdpZAWutWeJU="
-    },
     "regexpp": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/regexpp/-/regexpp-2.0.1.tgz",
@@ -4648,24 +4446,6 @@
       "integrity": "sha512-pb/MYmXstAkysRFx8piNI1tGFNQIFA3vkE3Gq4EuA1dF6gHp/+vgZqsCGJapvy8N3Q+4o7FwvquPJcnZ7RYy4g==",
       "dev": true
     },
-    "response": {
-      "version": "0.14.0",
-      "resolved": "https://registry.npmjs.org/response/-/response-0.14.0.tgz",
-      "integrity": "sha1-BmNS/z5rAm0EdYCUB2Y7Rob9JpY=",
-      "requires": {
-        "best-encoding": "^0.1.1",
-        "bl": "~0.7.0",
-        "caseless": "^0.3.0",
-        "mime": "~1.2.11"
-      },
-      "dependencies": {
-        "mime": {
-          "version": "1.2.11",
-          "resolved": "https://registry.npmjs.org/mime/-/mime-1.2.11.tgz",
-          "integrity": "sha1-WCA+7Ybjpe8XrtK32evUfwpg3RA="
-        }
-      }
-    },
     "restore-cursor": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/restore-cursor/-/restore-cursor-2.0.0.tgz",
@@ -4880,14 +4660,6 @@
         }
       }
     },
-    "sntp": {
-      "version": "0.1.4",
-      "resolved": "https://registry.npmjs.org/sntp/-/sntp-0.1.4.tgz",
-      "integrity": "sha1-XvSBuVGnspr/30r9fyaDj8ESD4Q=",
-      "requires": {
-        "hoek": "0.7.x"
-      }
-    },
     "source-map": {
       "version": "0.6.1",
       "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",
@@ -4975,49 +4747,11 @@
       "resolved": "https://registry.npmjs.org/stealthy-require/-/stealthy-require-1.1.1.tgz",
       "integrity": "sha1-NbCYdbT/SfJqd35QmzCQoyJr8ks="
     },
-    "stream-browserify": {
-      "version": "2.0.1",
-      "resolved": "https://registry.npmjs.org/stream-browserify/-/stream-browserify-2.0.1.tgz",
-      "integrity": "sha1-ZiZu5fm9uZQKTkUUyvtDu3Hlyds=",
-      "requires": {
-        "inherits": "~2.0.1",
-        "readable-stream": "^2.0.2"
-      },
-      "dependencies": {
-        "readable-stream": {
-          "version": "2.3.6",
-          "resolved": "http://registry.npmjs.org/readable-stream/-/readable-stream-2.3.6.tgz",
-          "integrity": "sha1-sRwn2IuP8fvgcGQ8+UsMea4bCq8=",
-          "requires": {
-            "core-util-is": "~1.0.0",
-            "inherits": "~2.0.3",
-            "isarray": "~1.0.0",
-            "process-nextick-args": "~2.0.0",
-            "safe-buffer": "~5.1.1",
-            "string_decoder": "~1.1.1",
-            "util-deprecate": "~1.0.1"
-          }
-        },
-        "string_decoder": {
-          "version": "1.1.1",
-          "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.1.1.tgz",
-          "integrity": "sha1-nPFhG6YmhdcDCunkujQUnDrwP8g=",
-          "requires": {
-            "safe-buffer": "~5.1.0"
-          }
-        }
-      }
-    },
     "stream-buffers": {
       "version": "0.2.6",
       "resolved": "https://registry.npmjs.org/stream-buffers/-/stream-buffers-0.2.6.tgz",
       "integrity": "sha1-GBwI1bs2kARfaUAbmuanoM8zE/w="
     },
-    "stream-counter": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/stream-counter/-/stream-counter-1.0.0.tgz",
-      "integrity": "sha1-kc8lac5NxQYf6816yyY5SloRR1E="
-    },
     "stream-meter": {
       "version": "1.0.4",
       "resolved": "https://registry.npmjs.org/stream-meter/-/stream-meter-1.0.4.tgz",
@@ -5055,6 +4789,12 @@
       "resolved": "https://registry.npmjs.org/stream-shift/-/stream-shift-1.0.0.tgz",
       "integrity": "sha1-1cdSgl5TZ+eG944Y5EXqIjoVWVI="
     },
+    "streamifier": {
+      "version": "0.1.1",
+      "resolved": "https://registry.npmjs.org/streamifier/-/streamifier-0.1.1.tgz",
+      "integrity": "sha1-l+mNj6TRBdYqJpHR3AfoINuN/E8=",
+      "dev": true
+    },
     "string-width": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/string-width/-/string-width-2.1.1.tgz",
@@ -5096,11 +4836,6 @@
         "function-bind": "^1.1.1"
       }
     },
-    "string_decoder": {
-      "version": "0.10.31",
-      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
-      "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ="
-    },
     "strip-ansi": {
       "version": "5.2.0",
       "resolved": "https://registry.npmjs.org/strip-ansi/-/strip-ansi-5.2.0.tgz",
@@ -5300,11 +5035,6 @@
       "integrity": "sha512-qOebF53frne81cf0S9B41ByenJ3/IuH8yJKngAX35CmiZySA0khhkovshKK+jGCaMnVomla7gVlIcc3EvKPbTQ==",
       "dev": true
     },
-    "tunnel-agent": {
-      "version": "0.2.0",
-      "resolved": "https://registry.npmjs.org/tunnel-agent/-/tunnel-agent-0.2.0.tgz",
-      "integrity": "sha1-aFPCr7GyEJ5FYp5JK9419Fnqaeg="
-    },
     "tweetnacl": {
       "version": "0.14.5",
       "resolved": "https://registry.npmjs.org/tweetnacl/-/tweetnacl-0.14.5.tgz",
diff --git a/services/filestore/package.json b/services/filestore/package.json
index 14e35cd8a2..6f1dde0e8a 100644
--- a/services/filestore/package.json
+++ b/services/filestore/package.json
@@ -28,21 +28,16 @@
     "fs-extra": "^1.0.0",
     "glob": "^7.1.6",
     "heapdump": "^0.3.2",
-    "knox": "~0.9.1",
     "logger-sharelatex": "^1.7.0",
     "metrics-sharelatex": "^2.2.0",
     "mocha": "5.2.0",
-    "node-transloadit": "0.0.4",
     "node-uuid": "~1.4.1",
     "pngcrush": "0.0.3",
     "range-parser": "^1.0.2",
-    "recluster": "^0.3.7",
     "request": "^2.88.0",
     "request-promise-native": "^1.0.8",
-    "response": "0.14.0",
     "rimraf": "2.2.8",
     "settings-sharelatex": "^1.1.0",
-    "stream-browserify": "^2.0.1",
     "stream-buffers": "~0.2.5",
     "stream-meter": "^1.0.4",
     "underscore": "~1.5.2"
@@ -68,6 +63,7 @@
     "prettier-eslint-cli": "^5.0.0",
     "sandboxed-module": "2.0.3",
     "sinon": "7.1.1",
-    "sinon-chai": "^3.3.0"
+    "sinon-chai": "^3.3.0",
+    "streamifier": "^0.1.1"
   }
 }
diff --git a/services/filestore/test/acceptance/js/FilestoreApp.js b/services/filestore/test/acceptance/js/FilestoreApp.js
index 718d53bcf8..20564e2d40 100644
--- a/services/filestore/test/acceptance/js/FilestoreApp.js
+++ b/services/filestore/test/acceptance/js/FilestoreApp.js
@@ -56,6 +56,7 @@ class FilestoreApp {
     }
 
     this.initing = false
+    this.persistor = require('../../../app/js/PersistorManager')
   }
 
   async waitForInit() {
diff --git a/services/filestore/test/acceptance/js/FilestoreTests.js b/services/filestore/test/acceptance/js/FilestoreTests.js
index d7dfbce57c..fd1baed474 100644
--- a/services/filestore/test/acceptance/js/FilestoreTests.js
+++ b/services/filestore/test/acceptance/js/FilestoreTests.js
@@ -11,6 +11,7 @@ const S3 = require('aws-sdk/clients/s3')
 const Stream = require('stream')
 const request = require('request')
 const { promisify } = require('util')
+const streamifier = require('streamifier')
 chai.use(require('chai-as-promised'))
 
 const fsWriteFile = promisify(fs.writeFile)
@@ -25,6 +26,20 @@ async function getMetric(filestoreUrl, metric) {
   return parseInt(found ? found[1] : 0) || 0
 }
 
+if (!process.env.AWS_ACCESS_KEY_ID) {
+  throw new Error('please provide credentials for the AWS S3 test server')
+}
+
+function streamToString(stream) {
+  const chunks = []
+  return new Promise((resolve, reject) => {
+    stream.on('data', chunk => chunks.push(chunk))
+    stream.on('error', reject)
+    stream.on('end', () => resolve(Buffer.concat(chunks).toString('utf8')))
+    stream.resume()
+  })
+}
+
 // store settings for multiple backends, so that we can test each one.
 // fs will always be available - add others if they are configured
 const BackendSettings = {
@@ -35,11 +50,8 @@ const BackendSettings = {
       public_files: Path.resolve(__dirname, '../../../public_files'),
       template_files: Path.resolve(__dirname, '../../../template_files')
     }
-  }
-}
-
-if (process.env.AWS_ACCESS_KEY_ID) {
-  BackendSettings.S3Persistor = {
+  },
+  S3Persistor: {
     backend: 's3',
     s3: {
       key: process.env.AWS_ACCESS_KEY_ID,
@@ -52,6 +64,62 @@ if (process.env.AWS_ACCESS_KEY_ID) {
       template_files: process.env.AWS_S3_TEMPLATE_FILES_BUCKET_NAME,
       public_files: process.env.AWS_S3_PUBLIC_FILES_BUCKET_NAME
     }
+  },
+  FallbackS3ToFSPersistor: {
+    backend: 's3',
+    s3: {
+      key: process.env.AWS_ACCESS_KEY_ID,
+      secret: process.env.AWS_SECRET_ACCESS_KEY,
+      endpoint: process.env.AWS_S3_ENDPOINT,
+      pathStyle: true
+    },
+    stores: {
+      user_files: process.env.AWS_S3_USER_FILES_BUCKET_NAME,
+      template_files: process.env.AWS_S3_TEMPLATE_FILES_BUCKET_NAME,
+      public_files: process.env.AWS_S3_PUBLIC_FILES_BUCKET_NAME
+    },
+    fallback: {
+      backend: 'fs',
+      buckets: {
+        [process.env.AWS_S3_USER_FILES_BUCKET_NAME]: Path.resolve(
+          __dirname,
+          '../../../user_files'
+        ),
+        [process.env.AWS_S3_PUBLIC_FILES_BUCKET_NAME]: Path.resolve(
+          __dirname,
+          '../../../public_files'
+        ),
+        [process.env.AWS_S3_TEMPLATE_FILES_BUCKET_NAME]: Path.resolve(
+          __dirname,
+          '../../../template_files'
+        )
+      }
+    }
+  },
+  FallbackFSToS3Persistor: {
+    backend: 'fs',
+    s3: {
+      key: process.env.AWS_ACCESS_KEY_ID,
+      secret: process.env.AWS_SECRET_ACCESS_KEY,
+      endpoint: process.env.AWS_S3_ENDPOINT,
+      pathStyle: true
+    },
+    stores: {
+      user_files: Path.resolve(__dirname, '../../../user_files'),
+      public_files: Path.resolve(__dirname, '../../../public_files'),
+      template_files: Path.resolve(__dirname, '../../../template_files')
+    },
+    fallback: {
+      backend: 's3',
+      buckets: {
+        [Path.resolve(__dirname, '../../../user_files')]: process.env
+          .AWS_S3_USER_FILES_BUCKET_NAME,
+        [Path.resolve(__dirname, '../../../public_files')]: process.env
+          .AWS_S3_PUBLIC_FILES_BUCKET_NAME,
+        [Path.resolve(__dirname, '../../../template_files')]: process.env
+          .AWS_S3_TEMPLATE_FILES_BUCKET_NAME
+      }
+    }
   }
 }
 
@@ -63,7 +131,7 @@ describe('Filestore', function() {
   // redefine the test suite for every available backend
   Object.keys(BackendSettings).forEach(backend => {
     describe(backend, function() {
-      let app, previousEgress, previousIngress
+      let app, previousEgress, previousIngress, projectId
 
       before(async function() {
         // create the app with the relevant filestore settings
@@ -84,6 +152,7 @@ describe('Filestore', function() {
             getMetric(filestoreUrl, 's3_ingress')
           ])
         }
+        projectId = `acceptance_tests_${Math.random()}`
       })
 
       it('should send a 200 for the status endpoint', async function() {
@@ -100,23 +169,21 @@ describe('Filestore', function() {
       })
 
       describe('with a file on the server', function() {
-        let fileId, fileUrl
+        let fileId, fileUrl, constantFileContent
 
         const localFileReadPath =
           '/tmp/filestore_acceptance_tests_file_read.txt'
-        const constantFileContent = [
-          'hello world',
-          `line 2 goes here ${Math.random()}`,
-          'there are 3 lines in all'
-        ].join('\n')
-
-        before(async function() {
-          await fsWriteFile(localFileReadPath, constantFileContent)
-        })
 
         beforeEach(async function() {
           fileId = Math.random()
-          fileUrl = `${filestoreUrl}/project/acceptance_tests/file/${directoryName}%2F${fileId}`
+          fileUrl = `${filestoreUrl}/project/${projectId}/file/${directoryName}%2F${fileId}`
+          constantFileContent = [
+            'hello world',
+            `line 2 goes here ${Math.random()}`,
+            'there are 3 lines in all'
+          ].join('\n')
+
+          await fsWriteFile(localFileReadPath, constantFileContent)
 
           const writeStream = request.post(fileUrl)
           const readStream = fs.createReadStream(localFileReadPath)
@@ -177,7 +244,7 @@ describe('Filestore', function() {
         })
 
         it('should be able to copy files', async function() {
-          const newProjectID = 'acceptance_tests_copyied_project'
+          const newProjectID = `acceptance_tests_copied_project_${Math.random()}`
           const newFileId = Math.random()
           const newFileUrl = `${filestoreUrl}/project/${newProjectID}/file/${directoryName}%2F${newFileId}`
           const opts = {
@@ -185,7 +252,7 @@ describe('Filestore', function() {
             uri: newFileUrl,
             json: {
               source: {
-                project_id: 'acceptance_tests',
+                project_id: projectId,
                 file_id: `${directoryName}/${fileId}`
               }
             }
@@ -198,6 +265,18 @@ describe('Filestore', function() {
           expect(response.body).to.equal(constantFileContent)
         })
 
+        it('should be able to overwrite the file', async function() {
+          const newContent = `here is some different content, ${Math.random()}`
+          const writeStream = request.post(fileUrl)
+          const readStream = streamifier.createReadStream(newContent)
+          // hack to consume the result to ensure the http request has been fully processed
+          const resultStream = fs.createWriteStream('/dev/null')
+          await pipeline(readStream, writeStream, resultStream)
+
+          const response = await rp.get(fileUrl)
+          expect(response.body).to.equal(newContent)
+        })
+
         if (backend === 'S3Persistor') {
           it('should record an egress metric for the upload', async function() {
             const metric = await getMetric(filestoreUrl, 's3_egress')
@@ -227,7 +306,7 @@ describe('Filestore', function() {
       })
 
       describe('with multiple files', function() {
-        let fileIds, fileUrls, project
+        let fileIds, fileUrls
         const directoryName = 'directory'
         const localFileReadPaths = [
           '/tmp/filestore_acceptance_tests_file_read_1.txt',
@@ -254,11 +333,10 @@ describe('Filestore', function() {
         })
 
         beforeEach(async function() {
-          project = `acceptance_tests_${Math.random()}`
           fileIds = [Math.random(), Math.random()]
           fileUrls = [
-            `${filestoreUrl}/project/${project}/file/${directoryName}%2F${fileIds[0]}`,
-            `${filestoreUrl}/project/${project}/file/${directoryName}%2F${fileIds[1]}`
+            `${filestoreUrl}/project/${projectId}/file/${directoryName}%2F${fileIds[0]}`,
+            `${filestoreUrl}/project/${projectId}/file/${directoryName}%2F${fileIds[1]}`
           ]
 
           const writeStreams = [
@@ -282,7 +360,7 @@ describe('Filestore', function() {
 
         it('should get the directory size', async function() {
           const response = await rp.get(
-            `${filestoreUrl}/project/${project}/size`
+            `${filestoreUrl}/project/${projectId}/size`
           )
           expect(parseInt(JSON.parse(response.body)['total bytes'])).to.equal(
             constantFileContents[0].length + constantFileContents[1].length
@@ -292,10 +370,10 @@ describe('Filestore', function() {
 
       if (backend === 'S3Persistor') {
         describe('with a file in a specific bucket', function() {
-          let constantFileContents, fileId, fileUrl, bucketName
+          let constantFileContent, fileId, fileUrl, bucketName
 
           beforeEach(async function() {
-            constantFileContents = `This is a file in a different S3 bucket ${Math.random()}`
+            constantFileContent = `This is a file in a different S3 bucket ${Math.random()}`
             fileId = Math.random().toString()
             bucketName = Math.random().toString()
             fileUrl = `${filestoreUrl}/bucket/${bucketName}/key/${fileId}`
@@ -320,14 +398,368 @@ describe('Filestore', function() {
               .upload({
                 Bucket: bucketName,
                 Key: fileId,
-                Body: constantFileContents
+                Body: constantFileContent
               })
               .promise()
           })
 
           it('should get the file from the specified bucket', async function() {
             const response = await rp.get(fileUrl)
-            expect(response.body).to.equal(constantFileContents)
+            expect(response.body).to.equal(constantFileContent)
+          })
+        })
+      }
+
+      if (BackendSettings[backend].fallback) {
+        describe('with a fallback', function() {
+          async function uploadStringToPersistor(
+            persistor,
+            bucket,
+            key,
+            content
+          ) {
+            const fileStream = streamifier.createReadStream(content)
+            await persistor.promises.sendStream(bucket, key, fileStream)
+          }
+
+          async function getStringFromPersistor(persistor, bucket, key) {
+            const stream = await persistor.promises.getFileStream(
+              bucket,
+              key,
+              {}
+            )
+            return streamToString(stream)
+          }
+
+          async function expectPersistorToHaveFile(
+            persistor,
+            bucket,
+            key,
+            content
+          ) {
+            const foundContent = await getStringFromPersistor(
+              persistor,
+              bucket,
+              key
+            )
+            expect(foundContent).to.equal(content)
+          }
+
+          async function expectPersistorNotToHaveFile(persistor, bucket, key) {
+            await expect(
+              getStringFromPersistor(persistor, bucket, key)
+            ).to.eventually.have.been.rejected.with.property(
+              'name',
+              'NotFoundError'
+            )
+          }
+
+          let constantFileContent,
+            fileId,
+            fileKey,
+            fileUrl,
+            bucket,
+            fallbackBucket
+
+          beforeEach(function() {
+            constantFileContent = `This is yet more file content ${Math.random()}`
+            fileId = Math.random().toString()
+            fileKey = `${projectId}/${directoryName}/${fileId}`
+            fileUrl = `${filestoreUrl}/project/${projectId}/file/${directoryName}%2F${fileId}`
+
+            bucket = Settings.filestore.stores.user_files
+            fallbackBucket = Settings.filestore.fallback.buckets[bucket]
+          })
+
+          describe('with a file in the fallback bucket', function() {
+            beforeEach(async function() {
+              await uploadStringToPersistor(
+                app.persistor.fallbackPersistor,
+                fallbackBucket,
+                fileKey,
+                constantFileContent
+              )
+            })
+
+            it('should not find file in the primary', async function() {
+              await expectPersistorNotToHaveFile(
+                app.persistor.primaryPersistor,
+                bucket,
+                fileKey
+              )
+            })
+
+            it('should find the file in the fallback', async function() {
+              await expectPersistorToHaveFile(
+                app.persistor.fallbackPersistor,
+                fallbackBucket,
+                fileKey,
+                constantFileContent
+              )
+            })
+
+            describe('when copyOnMiss is disabled', function() {
+              beforeEach(function() {
+                Settings.filestore.fallback.copyOnMiss = false
+              })
+
+              it('should fetch the file', async function() {
+                const res = await rp.get(fileUrl)
+                expect(res.body).to.equal(constantFileContent)
+              })
+
+              it('should not copy the file to the primary', async function() {
+                await rp.get(fileUrl)
+
+                await expectPersistorNotToHaveFile(
+                  app.persistor.primaryPersistor,
+                  bucket,
+                  fileKey
+                )
+              })
+            })
+
+            describe('when copyOnMiss is enabled', function() {
+              beforeEach(function() {
+                Settings.filestore.fallback.copyOnMiss = true
+              })
+
+              it('should fetch the file', async function() {
+                const res = await rp.get(fileUrl)
+                expect(res.body).to.equal(constantFileContent)
+              })
+
+              it('copies the file to the primary', async function() {
+                await rp.get(fileUrl)
+                // wait for the file to copy in the background
+                await promisify(setTimeout)(1000)
+
+                await expectPersistorToHaveFile(
+                  app.persistor.primaryPersistor,
+                  bucket,
+                  fileKey,
+                  constantFileContent
+                )
+              })
+            })
+
+            describe('when copying a file', function() {
+              let newFileId, newFileUrl, newFileKey, opts
+
+              beforeEach(function() {
+                const newProjectID = `acceptance_tests_copied_project_${Math.random()}`
+                newFileId = Math.random()
+                newFileUrl = `${filestoreUrl}/project/${newProjectID}/file/${directoryName}%2F${newFileId}`
+                newFileKey = `${newProjectID}/${directoryName}/${newFileId}`
+
+                opts = {
+                  method: 'put',
+                  uri: newFileUrl,
+                  json: {
+                    source: {
+                      project_id: projectId,
+                      file_id: `${directoryName}/${fileId}`
+                    }
+                  }
+                }
+              })
+
+              describe('when copyOnMiss is false', function() {
+                beforeEach(async function() {
+                  Settings.filestore.fallback.copyOnMiss = false
+
+                  const response = await rp(opts)
+                  expect(response.statusCode).to.equal(200)
+                })
+
+                it('should leave the old file in the old bucket', async function() {
+                  await expectPersistorToHaveFile(
+                    app.persistor.fallbackPersistor,
+                    fallbackBucket,
+                    fileKey,
+                    constantFileContent
+                  )
+                })
+
+                it('should not create a new file in the old bucket', async function() {
+                  await expectPersistorNotToHaveFile(
+                    app.persistor.fallbackPersistor,
+                    fallbackBucket,
+                    newFileKey
+                  )
+                })
+
+                it('should create a new file in the new bucket', async function() {
+                  await expectPersistorToHaveFile(
+                    app.persistor.primaryPersistor,
+                    bucket,
+                    newFileKey,
+                    constantFileContent
+                  )
+                })
+
+                it('should not copy the old file to the primary with the old key', async function() {
+                  // wait for the file to copy in the background
+                  await promisify(setTimeout)(1000)
+
+                  await expectPersistorNotToHaveFile(
+                    app.persistor.primaryPersistor,
+                    bucket,
+                    fileKey
+                  )
+                })
+              })
+
+              describe('when copyOnMiss is true', function() {
+                beforeEach(async function() {
+                  Settings.filestore.fallback.copyOnMiss = true
+
+                  const response = await rp(opts)
+                  expect(response.statusCode).to.equal(200)
+                })
+
+                it('should leave the old file in the old bucket', async function() {
+                  await expectPersistorToHaveFile(
+                    app.persistor.fallbackPersistor,
+                    fallbackBucket,
+                    fileKey,
+                    constantFileContent
+                  )
+                })
+
+                it('should not create a new file in the old bucket', async function() {
+                  await expectPersistorNotToHaveFile(
+                    app.persistor.fallbackPersistor,
+                    fallbackBucket,
+                    newFileKey
+                  )
+                })
+
+                it('should create a new file in the new bucket', async function() {
+                  await expectPersistorToHaveFile(
+                    app.persistor.primaryPersistor,
+                    bucket,
+                    newFileKey,
+                    constantFileContent
+                  )
+                })
+
+                it('should copy the old file to the primary with the old key', async function() {
+                  // wait for the file to copy in the background
+                  await promisify(setTimeout)(1000)
+
+                  await expectPersistorToHaveFile(
+                    app.persistor.primaryPersistor,
+                    bucket,
+                    fileKey,
+                    constantFileContent
+                  )
+                })
+              })
+            })
+          })
+
+          describe('when sending a file', function() {
+            beforeEach(async function() {
+              const writeStream = request.post(fileUrl)
+              const readStream = streamifier.createReadStream(
+                constantFileContent
+              )
+              // hack to consume the result to ensure the http request has been fully processed
+              const resultStream = fs.createWriteStream('/dev/null')
+              await pipeline(readStream, writeStream, resultStream)
+            })
+
+            it('should store the file on the primary', async function() {
+              await expectPersistorToHaveFile(
+                app.persistor.primaryPersistor,
+                bucket,
+                fileKey,
+                constantFileContent
+              )
+            })
+
+            it('should not store the file on the fallback', async function() {
+              await expectPersistorNotToHaveFile(
+                app.persistor.fallbackPersistor,
+                fallbackBucket,
+                `${projectId}/${directoryName}/${fileId}`
+              )
+            })
+          })
+
+          describe('when deleting a file', function() {
+            describe('when the file exists on the primary', function() {
+              beforeEach(async function() {
+                await uploadStringToPersistor(
+                  app.persistor.primaryPersistor,
+                  bucket,
+                  fileKey,
+                  constantFileContent
+                )
+              })
+
+              it('should delete the file', async function() {
+                const response = await rp.del(fileUrl)
+                expect(response.statusCode).to.equal(204)
+                await expect(
+                  rp.get(fileUrl)
+                ).to.eventually.be.rejected.and.have.property('statusCode', 404)
+              })
+            })
+
+            describe('when the file exists on the fallback', function() {
+              beforeEach(async function() {
+                await uploadStringToPersistor(
+                  app.persistor.fallbackPersistor,
+                  fallbackBucket,
+                  fileKey,
+                  constantFileContent
+                )
+              })
+
+              it('should delete the file', async function() {
+                const response = await rp.del(fileUrl)
+                expect(response.statusCode).to.equal(204)
+                await expect(
+                  rp.get(fileUrl)
+                ).to.eventually.be.rejected.and.have.property('statusCode', 404)
+              })
+            })
+
+            describe('when the file exists on both the primary and the fallback', function() {
+              beforeEach(async function() {
+                await uploadStringToPersistor(
+                  app.persistor.primaryPersistor,
+                  bucket,
+                  fileKey,
+                  constantFileContent
+                )
+                await uploadStringToPersistor(
+                  app.persistor.fallbackPersistor,
+                  fallbackBucket,
+                  fileKey,
+                  constantFileContent
+                )
+              })
+
+              it('should delete the files', async function() {
+                const response = await rp.del(fileUrl)
+                expect(response.statusCode).to.equal(204)
+                await expect(
+                  rp.get(fileUrl)
+                ).to.eventually.be.rejected.and.have.property('statusCode', 404)
+              })
+            })
+
+            describe('when the file does not exist', function() {
+              it('should return return 204', async function() {
+                // S3 doesn't give us a 404 when the object doesn't exist, so to stay
+                // consistent we merrily return 204 ourselves here as well
+                const response = await rp.del(fileUrl)
+                expect(response.statusCode).to.equal(204)
+              })
+            })
           })
         })
       }
@@ -341,7 +773,7 @@ describe('Filestore', function() {
 
         beforeEach(async function() {
           fileId = Math.random()
-          fileUrl = `${filestoreUrl}/project/acceptance_tests/file/${directoryName}%2F${fileId}`
+          fileUrl = `${filestoreUrl}/project/${projectId}/file/${directoryName}%2F${fileId}`
           const stat = await fsStat(localFileReadPath)
           localFileSize = stat.size
           const writeStream = request.post(fileUrl)
diff --git a/services/filestore/test/unit/js/FSPersistorManagerTests.js b/services/filestore/test/unit/js/FSPersistorTests.js
similarity index 69%
rename from services/filestore/test/unit/js/FSPersistorManagerTests.js
rename to services/filestore/test/unit/js/FSPersistorTests.js
index 3b3b4bf417..0a09869bc0 100644
--- a/services/filestore/test/unit/js/FSPersistorManagerTests.js
+++ b/services/filestore/test/unit/js/FSPersistorTests.js
@@ -7,24 +7,37 @@ const Errors = require('../../../app/js/Errors')
 chai.use(require('sinon-chai'))
 chai.use(require('chai-as-promised'))
 
-const modulePath = '../../../app/js/FSPersistorManager.js'
+const modulePath = '../../../app/js/FSPersistor.js'
 
-describe('FSPersistorManagerTests', function() {
+describe('FSPersistorTests', function() {
   const stat = { size: 4, isFile: sinon.stub().returns(true) }
   const fd = 1234
-  const readStream = 'readStream'
   const writeStream = 'writeStream'
   const remoteStream = 'remoteStream'
   const tempFile = '/tmp/potato.txt'
   const location = '/foo'
   const error = new Error('guru meditation error')
+  const md5 = 'ffffffff'
 
   const files = ['animals/wombat.tex', 'vegetables/potato.tex']
   const globs = [`${location}/${files[0]}`, `${location}/${files[1]}`]
   const filteredFilenames = ['animals_wombat.tex', 'vegetables_potato.tex']
-  let fs, rimraf, stream, LocalFileWriter, FSPersistorManager, glob
+  let fs,
+    rimraf,
+    stream,
+    LocalFileWriter,
+    FSPersistor,
+    glob,
+    readStream,
+    crypto,
+    Hash
 
   beforeEach(function() {
+    readStream = {
+      name: 'readStream',
+      on: sinon.stub().yields(),
+      pipe: sinon.stub()
+    }
     fs = {
       createReadStream: sinon.stub().returns(readStream),
       createWriteStream: sinon.stub().returns(writeStream),
@@ -41,14 +54,26 @@ describe('FSPersistorManagerTests', function() {
         deleteFile: sinon.stub().resolves()
       }
     }
-    FSPersistorManager = SandboxedModule.require(modulePath, {
+    Hash = {
+      end: sinon.stub(),
+      read: sinon.stub().returns(md5),
+      setEncoding: sinon.stub()
+    }
+    crypto = {
+      createHash: sinon.stub().returns(Hash)
+    }
+    FSPersistor = SandboxedModule.require(modulePath, {
       requires: {
         './LocalFileWriter': LocalFileWriter,
         './Errors': Errors,
         fs,
         glob,
         rimraf,
-        stream
+        stream,
+        crypto,
+        // imported by PersistorHelper but otherwise unused here
+        'stream-meter': {},
+        'logger-sharelatex': {}
       },
       globals: { console }
     })
@@ -57,7 +82,7 @@ describe('FSPersistorManagerTests', function() {
   describe('sendFile', function() {
     const localFilesystemPath = '/path/to/local/file'
     it('should copy the file', async function() {
-      await FSPersistorManager.promises.sendFile(
+      await FSPersistor.promises.sendFile(
         location,
         files[0],
         localFilesystemPath
@@ -72,33 +97,21 @@ describe('FSPersistorManagerTests', function() {
     it('should return an error if the file cannot be stored', async function() {
       stream.pipeline.yields(error)
       await expect(
-        FSPersistorManager.promises.sendFile(
-          location,
-          files[0],
-          localFilesystemPath
-        )
+        FSPersistor.promises.sendFile(location, files[0], localFilesystemPath)
       ).to.eventually.be.rejected.and.have.property('cause', error)
     })
   })
 
   describe('sendStream', function() {
     it('should send the stream to LocalFileWriter', async function() {
-      await FSPersistorManager.promises.sendStream(
-        location,
-        files[0],
-        remoteStream
-      )
+      await FSPersistor.promises.sendStream(location, files[0], remoteStream)
       expect(LocalFileWriter.promises.writeStream).to.have.been.calledWith(
         remoteStream
       )
     })
 
     it('should delete the temporary file', async function() {
-      await FSPersistorManager.promises.sendStream(
-        location,
-        files[0],
-        remoteStream
-      )
+      await FSPersistor.promises.sendStream(location, files[0], remoteStream)
       expect(LocalFileWriter.promises.deleteFile).to.have.been.calledWith(
         tempFile
       )
@@ -107,30 +120,55 @@ describe('FSPersistorManagerTests', function() {
     it('should return the error from LocalFileWriter', async function() {
       LocalFileWriter.promises.writeStream.rejects(error)
       await expect(
-        FSPersistorManager.promises.sendStream(location, files[0], remoteStream)
+        FSPersistor.promises.sendStream(location, files[0], remoteStream)
       ).to.eventually.be.rejectedWith(error)
     })
 
     it('should send the temporary file to the filestore', async function() {
-      await FSPersistorManager.promises.sendStream(
-        location,
-        files[0],
-        remoteStream
-      )
+      await FSPersistor.promises.sendStream(location, files[0], remoteStream)
       expect(fs.createReadStream).to.have.been.calledWith(tempFile)
     })
+
+    describe('when the md5 hash does not match', function() {
+      it('should return a write error', async function() {
+        await expect(
+          FSPersistor.promises.sendStream(
+            location,
+            files[0],
+            remoteStream,
+            '00000000'
+          )
+        )
+          .to.eventually.be.rejected.and.be.an.instanceOf(Errors.WriteError)
+          .and.have.property('message', 'md5 hash mismatch')
+      })
+
+      it('deletes the copied file', async function() {
+        try {
+          await FSPersistor.promises.sendStream(
+            location,
+            files[0],
+            remoteStream,
+            '00000000'
+          )
+        } catch (_) {}
+        expect(LocalFileWriter.promises.deleteFile).to.have.been.calledWith(
+          `${location}/${filteredFilenames[0]}`
+        )
+      })
+    })
   })
 
   describe('getFileStream', function() {
     it('should use correct file location', async function() {
-      await FSPersistorManager.promises.getFileStream(location, files[0], {})
+      await FSPersistor.promises.getFileStream(location, files[0], {})
       expect(fs.open).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}`
       )
     })
 
     it('should pass the options to createReadStream', async function() {
-      await FSPersistorManager.promises.getFileStream(location, files[0], {
+      await FSPersistor.promises.getFileStream(location, files[0], {
         start: 0,
         end: 8
       })
@@ -146,18 +184,14 @@ describe('FSPersistorManagerTests', function() {
       err.code = 'ENOENT'
       fs.open.yields(err)
 
-      await expect(
-        FSPersistorManager.promises.getFileStream(location, files[0], {})
-      )
+      await expect(FSPersistor.promises.getFileStream(location, files[0], {}))
         .to.eventually.be.rejected.and.be.an.instanceOf(Errors.NotFoundError)
         .and.have.property('cause', err)
     })
 
     it('should wrap any other error', async function() {
       fs.open.yields(error)
-      await expect(
-        FSPersistorManager.promises.getFileStream(location, files[0], {})
-      )
+      await expect(FSPersistor.promises.getFileStream(location, files[0], {}))
         .to.eventually.be.rejectedWith('failed to open file for streaming')
         .and.be.an.instanceOf(Errors.ReadError)
         .and.have.property('cause', error)
@@ -181,18 +215,18 @@ describe('FSPersistorManagerTests', function() {
 
     it('should return the file size', async function() {
       expect(
-        await FSPersistorManager.promises.getFileSize(location, files[0])
+        await FSPersistor.promises.getFileSize(location, files[0])
       ).to.equal(size)
     })
 
     it('should throw a NotFoundError if the file does not exist', async function() {
       await expect(
-        FSPersistorManager.promises.getFileSize(location, badFilename)
+        FSPersistor.promises.getFileSize(location, badFilename)
       ).to.eventually.be.rejected.and.be.an.instanceOf(Errors.NotFoundError)
     })
 
     it('should wrap any other error', async function() {
-      await expect(FSPersistorManager.promises.getFileSize(location, 'raccoon'))
+      await expect(FSPersistor.promises.getFileSize(location, 'raccoon'))
         .to.eventually.be.rejected.and.be.an.instanceOf(Errors.ReadError)
         .and.have.property('cause', error)
     })
@@ -200,28 +234,28 @@ describe('FSPersistorManagerTests', function() {
 
   describe('copyFile', function() {
     it('Should open the source for reading', async function() {
-      await FSPersistorManager.promises.copyFile(location, files[0], files[1])
+      await FSPersistor.promises.copyFile(location, files[0], files[1])
       expect(fs.createReadStream).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}`
       )
     })
 
     it('Should open the target for writing', async function() {
-      await FSPersistorManager.promises.copyFile(location, files[0], files[1])
+      await FSPersistor.promises.copyFile(location, files[0], files[1])
       expect(fs.createWriteStream).to.have.been.calledWith(
         `${location}/${filteredFilenames[1]}`
       )
     })
 
     it('Should pipe the source to the target', async function() {
-      await FSPersistorManager.promises.copyFile(location, files[0], files[1])
+      await FSPersistor.promises.copyFile(location, files[0], files[1])
       expect(stream.pipeline).to.have.been.calledWith(readStream, writeStream)
     })
   })
 
   describe('deleteFile', function() {
     it('Should call unlink with correct options', async function() {
-      await FSPersistorManager.promises.deleteFile(location, files[0])
+      await FSPersistor.promises.deleteFile(location, files[0])
       expect(fs.unlink).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}`
       )
@@ -230,14 +264,14 @@ describe('FSPersistorManagerTests', function() {
     it('Should propagate the error', async function() {
       fs.unlink.yields(error)
       await expect(
-        FSPersistorManager.promises.deleteFile(location, files[0])
+        FSPersistor.promises.deleteFile(location, files[0])
       ).to.eventually.be.rejected.and.have.property('cause', error)
     })
   })
 
   describe('deleteDirectory', function() {
     it('Should call rmdir(rimraf) with correct options', async function() {
-      await FSPersistorManager.promises.deleteDirectory(location, files[0])
+      await FSPersistor.promises.deleteDirectory(location, files[0])
       expect(rimraf).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}`
       )
@@ -246,7 +280,7 @@ describe('FSPersistorManagerTests', function() {
     it('Should propagate the error', async function() {
       rimraf.yields(error)
       await expect(
-        FSPersistorManager.promises.deleteDirectory(location, files[0])
+        FSPersistor.promises.deleteDirectory(location, files[0])
       ).to.eventually.be.rejected.and.have.property('cause', error)
     })
   })
@@ -266,7 +300,7 @@ describe('FSPersistorManagerTests', function() {
     })
 
     it('Should call stat with correct options', async function() {
-      await FSPersistorManager.promises.checkIfFileExists(location, files[0])
+      await FSPersistor.promises.checkIfFileExists(location, files[0])
       expect(fs.stat).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}`
       )
@@ -274,23 +308,18 @@ describe('FSPersistorManagerTests', function() {
 
     it('Should return true for existing files', async function() {
       expect(
-        await FSPersistorManager.promises.checkIfFileExists(location, files[0])
+        await FSPersistor.promises.checkIfFileExists(location, files[0])
       ).to.equal(true)
     })
 
     it('Should return false for non-existing files', async function() {
       expect(
-        await FSPersistorManager.promises.checkIfFileExists(
-          location,
-          badFilename
-        )
+        await FSPersistor.promises.checkIfFileExists(location, badFilename)
       ).to.equal(false)
     })
 
     it('should wrap the error if there is a problem', async function() {
-      await expect(
-        FSPersistorManager.promises.checkIfFileExists(location, 'llama')
-      )
+      await expect(FSPersistor.promises.checkIfFileExists(location, 'llama'))
         .to.eventually.be.rejected.and.be.an.instanceOf(Errors.ReadError)
         .and.have.property('cause', error)
     })
@@ -299,9 +328,7 @@ describe('FSPersistorManagerTests', function() {
   describe('directorySize', function() {
     it('should wrap the error', async function() {
       glob.yields(error)
-      await expect(
-        FSPersistorManager.promises.directorySize(location, files[0])
-      )
+      await expect(FSPersistor.promises.directorySize(location, files[0]))
         .to.eventually.be.rejected.and.be.an.instanceOf(Errors.ReadError)
         .and.include({ cause: error })
         .and.have.property('info')
@@ -309,7 +336,7 @@ describe('FSPersistorManagerTests', function() {
     })
 
     it('should filter the directory name', async function() {
-      await FSPersistorManager.promises.directorySize(location, files[0])
+      await FSPersistor.promises.directorySize(location, files[0])
       expect(glob).to.have.been.calledWith(
         `${location}/${filteredFilenames[0]}_*`
       )
@@ -317,7 +344,7 @@ describe('FSPersistorManagerTests', function() {
 
     it('should sum directory files size', async function() {
       expect(
-        await FSPersistorManager.promises.directorySize(location, files[0])
+        await FSPersistor.promises.directorySize(location, files[0])
       ).to.equal(stat.size * files.length)
     })
   })
diff --git a/services/filestore/test/unit/js/MigrationPersistorTests.js b/services/filestore/test/unit/js/MigrationPersistorTests.js
new file mode 100644
index 0000000000..db8401c78c
--- /dev/null
+++ b/services/filestore/test/unit/js/MigrationPersistorTests.js
@@ -0,0 +1,519 @@
+const sinon = require('sinon')
+const chai = require('chai')
+const { expect } = chai
+const modulePath = '../../../app/js/MigrationPersistor.js'
+const SandboxedModule = require('sandboxed-module')
+
+const Errors = require('../../../app/js/Errors')
+
+// Not all methods are tested here, but a method with each type of wrapping has
+// tests. Specifically, the following wrapping methods are tested here:
+// getFileStream: _wrapFallbackMethod
+// sendStream: forward-to-primary
+// deleteFile: _wrapMethodOnBothPersistors
+// copyFile: copyFileWithFallback
+
+describe('MigrationPersistorTests', function() {
+  const bucket = 'womBucket'
+  const fallbackBucket = 'bucKangaroo'
+  const key = 'monKey'
+  const destKey = 'donKey'
+  const genericError = new Error('guru meditation error')
+  const notFoundError = new Errors.NotFoundError('not found')
+  const size = 33
+  const md5 = 'ffffffff'
+
+  let Metrics,
+    Settings,
+    Logger,
+    Stream,
+    MigrationPersistor,
+    fileStream,
+    newPersistor
+
+  beforeEach(function() {
+    fileStream = {
+      name: 'fileStream',
+      on: sinon
+        .stub()
+        .withArgs('end')
+        .yields(),
+      pipe: sinon.stub()
+    }
+
+    newPersistor = function(hasFile) {
+      return {
+        promises: {
+          sendFile: sinon.stub().resolves(),
+          sendStream: sinon.stub().resolves(),
+          getFileStream: hasFile
+            ? sinon.stub().resolves(fileStream)
+            : sinon.stub().rejects(notFoundError),
+          deleteDirectory: sinon.stub().resolves(),
+          getFileSize: hasFile
+            ? sinon.stub().resolves(size)
+            : sinon.stub().rejects(notFoundError),
+          deleteFile: sinon.stub().resolves(),
+          copyFile: hasFile
+            ? sinon.stub().resolves()
+            : sinon.stub().rejects(notFoundError),
+          checkIfFileExists: sinon.stub().resolves(hasFile),
+          directorySize: hasFile
+            ? sinon.stub().resolves(size)
+            : sinon.stub().rejects(notFoundError),
+          getFileMd5Hash: hasFile
+            ? sinon.stub().resolves(md5)
+            : sinon.stub().rejects(notFoundError)
+        }
+      }
+    }
+
+    Settings = {
+      filestore: {
+        fallback: {
+          buckets: {
+            [bucket]: fallbackBucket
+          }
+        }
+      }
+    }
+
+    Metrics = {
+      inc: sinon.stub()
+    }
+
+    Stream = {
+      pipeline: sinon.stub().yields(),
+      PassThrough: sinon.stub()
+    }
+
+    Logger = {
+      warn: sinon.stub()
+    }
+
+    MigrationPersistor = SandboxedModule.require(modulePath, {
+      requires: {
+        'settings-sharelatex': Settings,
+        stream: Stream,
+        './Errors': Errors,
+        'metrics-sharelatex': Metrics,
+        'logger-sharelatex': Logger
+      },
+      globals: { console }
+    })
+  })
+
+  describe('getFileStream', function() {
+    const options = { wombat: 'potato' }
+    describe('when the primary persistor has the file', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor, response
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(true)
+        fallbackPersistor = newPersistor(false)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        response = await migrationPersistor.promises.getFileStream(
+          bucket,
+          key,
+          options
+        )
+      })
+
+      it('should return the file stream', function() {
+        expect(response).to.equal(fileStream)
+      })
+
+      it('should fetch the file from the primary persistor, with the correct options', function() {
+        expect(
+          primaryPersistor.promises.getFileStream
+        ).to.have.been.calledWithExactly(bucket, key, options)
+      })
+
+      it('should not query the fallback persistor', function() {
+        expect(fallbackPersistor.promises.getFileStream).not.to.have.been.called
+      })
+    })
+
+    describe('when the fallback persistor has the file', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor, response
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(true)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        response = await migrationPersistor.promises.getFileStream(
+          bucket,
+          key,
+          options
+        )
+      })
+
+      it('should return the file stream', function() {
+        expect(response).to.be.an.instanceOf(Stream.PassThrough)
+      })
+
+      it('should fetch the file from the primary persistor with the correct options', function() {
+        expect(
+          primaryPersistor.promises.getFileStream
+        ).to.have.been.calledWithExactly(bucket, key, options)
+      })
+
+      it('should fetch the file from the fallback persistor with the fallback bucket with the correct options', function() {
+        expect(
+          fallbackPersistor.promises.getFileStream
+        ).to.have.been.calledWithExactly(fallbackBucket, key, options)
+      })
+
+      it('should create one read stream', function() {
+        expect(fallbackPersistor.promises.getFileStream).to.have.been.calledOnce
+      })
+
+      it('should not send the file to the primary', function() {
+        expect(primaryPersistor.promises.sendStream).not.to.have.been.called
+      })
+    })
+
+    describe('when the file should be copied to the primary', function() {
+      let primaryPersistor,
+        fallbackPersistor,
+        migrationPersistor,
+        returnedStream
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(true)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        Settings.filestore.fallback.copyOnMiss = true
+        returnedStream = await migrationPersistor.promises.getFileStream(
+          bucket,
+          key,
+          options
+        )
+      })
+
+      it('should create one read stream', function() {
+        expect(fallbackPersistor.promises.getFileStream).to.have.been.calledOnce
+      })
+
+      it('should get the md5 hash from the source', function() {
+        expect(
+          fallbackPersistor.promises.getFileMd5Hash
+        ).to.have.been.calledWith(fallbackBucket, key)
+      })
+
+      it('should send a stream to the primary', function() {
+        expect(
+          primaryPersistor.promises.sendStream
+        ).to.have.been.calledWithExactly(
+          bucket,
+          key,
+          sinon.match.instanceOf(Stream.PassThrough),
+          md5
+        )
+      })
+
+      it('should send a stream to the client', function() {
+        expect(returnedStream).to.be.an.instanceOf(Stream.PassThrough)
+      })
+    })
+
+    describe('when neither persistor has the file', function() {
+      it('rejects with a NotFoundError', async function() {
+        const migrationPersistor = MigrationPersistor(
+          newPersistor(false),
+          newPersistor(false)
+        )
+        return expect(
+          migrationPersistor.promises.getFileStream(bucket, key)
+        ).to.eventually.be.rejected.and.be.an.instanceOf(Errors.NotFoundError)
+      })
+    })
+
+    describe('when the primary persistor throws an unexpected error', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor, error
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(true)
+        primaryPersistor.promises.getFileStream = sinon
+          .stub()
+          .rejects(genericError)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        try {
+          await migrationPersistor.promises.getFileStream(bucket, key, options)
+        } catch (err) {
+          error = err
+        }
+      })
+
+      it('rejects with the error', function() {
+        expect(error).to.equal(genericError)
+      })
+
+      it('does not call the fallback', function() {
+        expect(fallbackPersistor.promises.getFileStream).not.to.have.been.called
+      })
+    })
+
+    describe('when the fallback persistor throws an unexpected error', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor, error
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(false)
+        fallbackPersistor.promises.getFileStream = sinon
+          .stub()
+          .rejects(genericError)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        try {
+          await migrationPersistor.promises.getFileStream(bucket, key, options)
+        } catch (err) {
+          error = err
+        }
+      })
+
+      it('rejects with the error', function() {
+        expect(error).to.equal(genericError)
+      })
+
+      it('should have called the fallback', function() {
+        expect(
+          fallbackPersistor.promises.getFileStream
+        ).to.have.been.calledWith(fallbackBucket, key)
+      })
+    })
+  })
+
+  describe('sendStream', function() {
+    let primaryPersistor, fallbackPersistor, migrationPersistor
+    beforeEach(function() {
+      primaryPersistor = newPersistor(false)
+      fallbackPersistor = newPersistor(false)
+      migrationPersistor = MigrationPersistor(
+        primaryPersistor,
+        fallbackPersistor
+      )
+    })
+
+    describe('when it works', function() {
+      beforeEach(async function() {
+        return migrationPersistor.promises.sendStream(bucket, key, fileStream)
+      })
+
+      it('should send the file to the primary persistor', function() {
+        expect(
+          primaryPersistor.promises.sendStream
+        ).to.have.been.calledWithExactly(bucket, key, fileStream)
+      })
+
+      it('should not send the file to the fallback persistor', function() {
+        expect(fallbackPersistor.promises.sendStream).not.to.have.been.called
+      })
+    })
+
+    describe('when the primary persistor throws an error', function() {
+      it('returns the error', async function() {
+        primaryPersistor.promises.sendStream.rejects(notFoundError)
+        return expect(
+          migrationPersistor.promises.sendStream(bucket, key, fileStream)
+        ).to.eventually.be.rejected.and.be.an.instanceOf(Errors.NotFoundError)
+      })
+    })
+  })
+
+  describe('deleteFile', function() {
+    let primaryPersistor, fallbackPersistor, migrationPersistor
+    beforeEach(function() {
+      primaryPersistor = newPersistor(false)
+      fallbackPersistor = newPersistor(false)
+      migrationPersistor = MigrationPersistor(
+        primaryPersistor,
+        fallbackPersistor
+      )
+    })
+
+    describe('when it works', function() {
+      beforeEach(async function() {
+        return migrationPersistor.promises.deleteFile(bucket, key)
+      })
+
+      it('should delete the file from the primary', function() {
+        expect(
+          primaryPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(bucket, key)
+      })
+
+      it('should delete the file from the fallback', function() {
+        expect(
+          fallbackPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(fallbackBucket, key)
+      })
+    })
+
+    describe('when the primary persistor throws an error', function() {
+      let error
+      beforeEach(async function() {
+        primaryPersistor.promises.deleteFile.rejects(genericError)
+        try {
+          await migrationPersistor.promises.deleteFile(bucket, key)
+        } catch (err) {
+          error = err
+        }
+      })
+
+      it('should return the error', function() {
+        expect(error).to.equal(genericError)
+      })
+
+      it('should delete the file from the primary', function() {
+        expect(
+          primaryPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(bucket, key)
+      })
+
+      it('should delete the file from the fallback', function() {
+        expect(
+          fallbackPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(fallbackBucket, key)
+      })
+    })
+
+    describe('when the fallback persistor throws an error', function() {
+      let error
+      beforeEach(async function() {
+        fallbackPersistor.promises.deleteFile.rejects(genericError)
+        try {
+          await migrationPersistor.promises.deleteFile(bucket, key)
+        } catch (err) {
+          error = err
+        }
+      })
+
+      it('should return the error', function() {
+        expect(error).to.equal(genericError)
+      })
+
+      it('should delete the file from the primary', function() {
+        expect(
+          primaryPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(bucket, key)
+      })
+
+      it('should delete the file from the fallback', function() {
+        expect(
+          fallbackPersistor.promises.deleteFile
+        ).to.have.been.calledWithExactly(fallbackBucket, key)
+      })
+    })
+  })
+
+  describe('copyFile', function() {
+    describe('when the file exists on the primary', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(true)
+        fallbackPersistor = newPersistor(false)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        return migrationPersistor.promises.copyFile(bucket, key, destKey)
+      })
+
+      it('should call copyFile to copy the file', function() {
+        expect(
+          primaryPersistor.promises.copyFile
+        ).to.have.been.calledWithExactly(bucket, key, destKey)
+      })
+
+      it('should not try to read from the fallback', function() {
+        expect(fallbackPersistor.promises.getFileStream).not.to.have.been.called
+      })
+    })
+
+    describe('when the file does not exist on the primary', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(true)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        return migrationPersistor.promises.copyFile(bucket, key, destKey)
+      })
+
+      it('should call copyFile to copy the file', function() {
+        expect(
+          primaryPersistor.promises.copyFile
+        ).to.have.been.calledWithExactly(bucket, key, destKey)
+      })
+
+      it('should fetch the file from the fallback', function() {
+        expect(
+          fallbackPersistor.promises.getFileStream
+        ).not.to.have.been.calledWithExactly(fallbackBucket, key)
+      })
+
+      it('should get the md5 hash from the source', function() {
+        expect(
+          fallbackPersistor.promises.getFileMd5Hash
+        ).to.have.been.calledWith(fallbackBucket, key)
+      })
+
+      it('should send the file to the primary', function() {
+        expect(
+          primaryPersistor.promises.sendStream
+        ).to.have.been.calledWithExactly(
+          bucket,
+          destKey,
+          sinon.match.instanceOf(Stream.PassThrough),
+          md5
+        )
+      })
+    })
+
+    describe('when the file does not exist on the fallback', function() {
+      let primaryPersistor, fallbackPersistor, migrationPersistor, error
+      beforeEach(async function() {
+        primaryPersistor = newPersistor(false)
+        fallbackPersistor = newPersistor(false)
+        migrationPersistor = MigrationPersistor(
+          primaryPersistor,
+          fallbackPersistor
+        )
+        try {
+          await migrationPersistor.promises.copyFile(bucket, key, destKey)
+        } catch (err) {
+          error = err
+        }
+      })
+
+      it('should call copyFile to copy the file', function() {
+        expect(
+          primaryPersistor.promises.copyFile
+        ).to.have.been.calledWithExactly(bucket, key, destKey)
+      })
+
+      it('should fetch the file from the fallback', function() {
+        expect(
+          fallbackPersistor.promises.getFileStream
+        ).not.to.have.been.calledWithExactly(fallbackBucket, key)
+      })
+
+      it('should return a not-found error', function() {
+        expect(error).to.be.an.instanceOf(Errors.NotFoundError)
+      })
+    })
+  })
+})
diff --git a/services/filestore/test/unit/js/PersistorManagerTests.js b/services/filestore/test/unit/js/PersistorManagerTests.js
index 0ecbb22078..cdc9de0f92 100644
--- a/services/filestore/test/unit/js/PersistorManagerTests.js
+++ b/services/filestore/test/unit/js/PersistorManagerTests.js
@@ -6,18 +6,14 @@ const SandboxedModule = require('sandboxed-module')
 const modulePath = '../../../app/js/PersistorManager.js'
 
 describe('PersistorManager', function() {
-  let PersistorManager,
-    FSPersistorManager,
-    S3PersistorManager,
-    settings,
-    requires
+  let PersistorManager, FSPersistor, S3Persistor, settings, requires
 
   beforeEach(function() {
-    FSPersistorManager = {
-      wrappedMethod: sinon.stub().returns('FSPersistorManager')
+    FSPersistor = {
+      wrappedMethod: sinon.stub().returns('FSPersistor')
     }
-    S3PersistorManager = {
-      wrappedMethod: sinon.stub().returns('S3PersistorManager')
+    S3Persistor = {
+      wrappedMethod: sinon.stub().returns('S3Persistor')
     }
 
     settings = {
@@ -25,8 +21,8 @@ describe('PersistorManager', function() {
     }
 
     requires = {
-      './S3PersistorManager': S3PersistorManager,
-      './FSPersistorManager': FSPersistorManager,
+      './S3Persistor': S3Persistor,
+      './FSPersistor': FSPersistor,
       'settings-sharelatex': settings,
       'logger-sharelatex': {
         log() {},
@@ -40,7 +36,7 @@ describe('PersistorManager', function() {
     PersistorManager = SandboxedModule.require(modulePath, { requires })
 
     expect(PersistorManager).to.respondTo('wrappedMethod')
-    expect(PersistorManager.wrappedMethod()).to.equal('S3PersistorManager')
+    expect(PersistorManager.wrappedMethod()).to.equal('S3Persistor')
   })
 
   it("should implement the S3 wrapped method when 'aws-sdk' is configured", function() {
@@ -48,7 +44,7 @@ describe('PersistorManager', function() {
     PersistorManager = SandboxedModule.require(modulePath, { requires })
 
     expect(PersistorManager).to.respondTo('wrappedMethod')
-    expect(PersistorManager.wrappedMethod()).to.equal('S3PersistorManager')
+    expect(PersistorManager.wrappedMethod()).to.equal('S3Persistor')
   })
 
   it('should implement the FS wrapped method when FS is configured', function() {
@@ -56,7 +52,7 @@ describe('PersistorManager', function() {
     PersistorManager = SandboxedModule.require(modulePath, { requires })
 
     expect(PersistorManager).to.respondTo('wrappedMethod')
-    expect(PersistorManager.wrappedMethod()).to.equal('FSPersistorManager')
+    expect(PersistorManager.wrappedMethod()).to.equal('FSPersistor')
   })
 
   it('should throw an error when the backend is not configured', function() {
diff --git a/services/filestore/test/unit/js/S3PersistorManagerTests.js b/services/filestore/test/unit/js/S3PersistorTests.js
similarity index 78%
rename from services/filestore/test/unit/js/S3PersistorManagerTests.js
rename to services/filestore/test/unit/js/S3PersistorTests.js
index daeac66d3f..9686deed5f 100644
--- a/services/filestore/test/unit/js/S3PersistorManagerTests.js
+++ b/services/filestore/test/unit/js/S3PersistorTests.js
@@ -1,12 +1,12 @@
 const sinon = require('sinon')
 const chai = require('chai')
 const { expect } = chai
-const modulePath = '../../../app/js/S3PersistorManager.js'
+const modulePath = '../../../app/js/S3Persistor.js'
 const SandboxedModule = require('sandboxed-module')
 
 const Errors = require('../../../app/js/Errors')
 
-describe('S3PersistorManagerTests', function() {
+describe('S3PersistorTests', function() {
   const defaultS3Key = 'frog'
   const defaultS3Secret = 'prince'
   const defaultS3Credentials = {
@@ -26,21 +26,26 @@ describe('S3PersistorManagerTests', function() {
     { Key: 'hippo', Size: 22 }
   ]
   const filesSize = 33
+  const md5 = 'ffffffff00000000ffffffff00000000'
 
   let Metrics,
+    Logger,
     S3,
     Fs,
     Meter,
     MeteredStream,
     ReadStream,
-    S3PersistorManager,
+    Stream,
+    S3Persistor,
     S3Client,
     S3ReadStream,
     S3NotFoundError,
     S3AccessDeniedError,
     FileNotFoundError,
     EmptyPromise,
-    settings
+    settings,
+    Hash,
+    crypto
 
   beforeEach(function() {
     settings = {
@@ -56,6 +61,10 @@ describe('S3PersistorManagerTests', function() {
       }
     }
 
+    Stream = {
+      pipeline: sinon.stub().yields()
+    }
+
     EmptyPromise = {
       promise: sinon.stub().resolves()
     }
@@ -65,7 +74,11 @@ describe('S3PersistorManagerTests', function() {
     }
 
     ReadStream = {
-      pipe: sinon.stub().returns('readStream')
+      pipe: sinon.stub().returns('readStream'),
+      on: sinon
+        .stub()
+        .withArgs('end')
+        .yields()
     }
 
     FileNotFoundError = new Error('File not found')
@@ -76,6 +89,7 @@ describe('S3PersistorManagerTests', function() {
     }
 
     MeteredStream = {
+      type: 'metered',
       on: sinon.stub(),
       bytes: objectSize
     }
@@ -90,7 +104,7 @@ describe('S3PersistorManagerTests', function() {
 
     S3ReadStream = {
       on: sinon.stub(),
-      pipe: sinon.stub().returns('s3Stream'),
+      pipe: sinon.stub(),
       removeListener: sinon.stub()
     }
     S3ReadStream.on.withArgs('readable').yields()
@@ -100,7 +114,8 @@ describe('S3PersistorManagerTests', function() {
       }),
       headObject: sinon.stub().returns({
         promise: sinon.stub().resolves({
-          ContentLength: objectSize
+          ContentLength: objectSize,
+          ETag: md5
         })
       }),
       listObjects: sinon.stub().returns({
@@ -108,21 +123,39 @@ describe('S3PersistorManagerTests', function() {
           Contents: files
         })
       }),
-      upload: sinon.stub().returns(EmptyPromise),
+      upload: sinon
+        .stub()
+        .returns({ promise: sinon.stub().resolves({ ETag: `"${md5}"` }) }),
       copyObject: sinon.stub().returns(EmptyPromise),
       deleteObject: sinon.stub().returns(EmptyPromise),
       deleteObjects: sinon.stub().returns(EmptyPromise)
     }
     S3 = sinon.stub().returns(S3Client)
 
-    S3PersistorManager = SandboxedModule.require(modulePath, {
+    Hash = {
+      end: sinon.stub(),
+      read: sinon.stub().returns(md5),
+      setEncoding: sinon.stub()
+    }
+    crypto = {
+      createHash: sinon.stub().returns(Hash)
+    }
+
+    Logger = {
+      warn: sinon.stub()
+    }
+
+    S3Persistor = SandboxedModule.require(modulePath, {
       requires: {
         'aws-sdk/clients/s3': S3,
         'settings-sharelatex': settings,
+        'logger-sharelatex': Logger,
         './Errors': Errors,
         fs: Fs,
         'stream-meter': Meter,
-        'metrics-sharelatex': Metrics
+        stream: Stream,
+        'metrics-sharelatex': Metrics,
+        crypto
       },
       globals: { console }
     })
@@ -133,11 +166,11 @@ describe('S3PersistorManagerTests', function() {
       let stream
 
       beforeEach(async function() {
-        stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+        stream = await S3Persistor.promises.getFileStream(bucket, key)
       })
 
-      it('returns a stream', function() {
-        expect(stream).to.equal('s3Stream')
+      it('returns a metered stream', function() {
+        expect(stream).to.equal(MeteredStream)
       })
 
       it('sets the AWS client up with credentials from settings', function() {
@@ -152,7 +185,10 @@ describe('S3PersistorManagerTests', function() {
       })
 
       it('pipes the stream through the meter', function() {
-        expect(S3ReadStream.pipe).to.have.been.calledWith(MeteredStream)
+        expect(Stream.pipeline).to.have.been.calledWith(
+          S3ReadStream,
+          MeteredStream
+        )
       })
 
       it('records an ingress metric', function() {
@@ -164,14 +200,14 @@ describe('S3PersistorManagerTests', function() {
       let stream
 
       beforeEach(async function() {
-        stream = await S3PersistorManager.promises.getFileStream(bucket, key, {
+        stream = await S3Persistor.promises.getFileStream(bucket, key, {
           start: 5,
           end: 10
         })
       })
 
-      it('returns a stream', function() {
-        expect(stream).to.equal('s3Stream')
+      it('returns a metered stream', function() {
+        expect(stream).to.equal(MeteredStream)
       })
 
       it('passes the byte range on to S3', function() {
@@ -201,11 +237,11 @@ describe('S3PersistorManagerTests', function() {
           auth_secret: alternativeSecret
         }
 
-        stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+        stream = await S3Persistor.promises.getFileStream(bucket, key)
       })
 
-      it('returns a stream', function() {
-        expect(stream).to.equal('s3Stream')
+      it('returns a metered stream', function() {
+        expect(stream).to.equal(MeteredStream)
       })
 
       it('sets the AWS client up with the alternative credentials', function() {
@@ -220,16 +256,13 @@ describe('S3PersistorManagerTests', function() {
       })
 
       it('caches the credentials', async function() {
-        stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+        stream = await S3Persistor.promises.getFileStream(bucket, key)
 
         expect(S3).to.have.been.calledOnceWith(alternativeS3Credentials)
       })
 
       it('uses the default credentials for an unknown bucket', async function() {
-        stream = await S3PersistorManager.promises.getFileStream(
-          'anotherBucket',
-          key
-        )
+        stream = await S3Persistor.promises.getFileStream('anotherBucket', key)
 
         expect(S3).to.have.been.calledTwice
         expect(S3.firstCall).to.have.been.calledWith(alternativeS3Credentials)
@@ -237,14 +270,8 @@ describe('S3PersistorManagerTests', function() {
       })
 
       it('caches the default credentials', async function() {
-        stream = await S3PersistorManager.promises.getFileStream(
-          'anotherBucket',
-          key
-        )
-        stream = await S3PersistorManager.promises.getFileStream(
-          'anotherBucket',
-          key
-        )
+        stream = await S3Persistor.promises.getFileStream('anotherBucket', key)
+        stream = await S3Persistor.promises.getFileStream('anotherBucket', key)
 
         expect(S3).to.have.been.calledTwice
         expect(S3.firstCall).to.have.been.calledWith(alternativeS3Credentials)
@@ -256,7 +283,7 @@ describe('S3PersistorManagerTests', function() {
         delete settings.filestore.s3.secret
 
         await expect(
-          S3PersistorManager.promises.getFileStream('anotherBucket', key)
+          S3Persistor.promises.getFileStream('anotherBucket', key)
         ).to.eventually.be.rejected.and.be.an.instanceOf(Errors.SettingsError)
       })
     })
@@ -268,7 +295,7 @@ describe('S3PersistorManagerTests', function() {
         S3ReadStream.on = sinon.stub()
         S3ReadStream.on.withArgs('error').yields(S3NotFoundError)
         try {
-          stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+          stream = await S3Persistor.promises.getFileStream(bucket, key)
         } catch (err) {
           error = err
         }
@@ -282,12 +309,12 @@ describe('S3PersistorManagerTests', function() {
         expect(error).to.be.an.instanceOf(Errors.NotFoundError)
       })
 
-      it('wraps the error from S3', function() {
-        expect(error.cause).to.equal(S3NotFoundError)
+      it('wraps the error', function() {
+        expect(error.cause).to.exist
       })
 
       it('stores the bucket and key in the error', function() {
-        expect(error.info).to.deep.equal({ Bucket: bucket, Key: key })
+        expect(error.info).to.include({ bucketName: bucket, key: key })
       })
     })
 
@@ -298,7 +325,7 @@ describe('S3PersistorManagerTests', function() {
         S3ReadStream.on = sinon.stub()
         S3ReadStream.on.withArgs('error').yields(S3AccessDeniedError)
         try {
-          stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+          stream = await S3Persistor.promises.getFileStream(bucket, key)
         } catch (err) {
           error = err
         }
@@ -312,12 +339,12 @@ describe('S3PersistorManagerTests', function() {
         expect(error).to.be.an.instanceOf(Errors.NotFoundError)
       })
 
-      it('wraps the error from S3', function() {
-        expect(error.cause).to.equal(S3AccessDeniedError)
+      it('wraps the error', function() {
+        expect(error.cause).to.exist
       })
 
       it('stores the bucket and key in the error', function() {
-        expect(error.info).to.deep.equal({ Bucket: bucket, Key: key })
+        expect(error.info).to.include({ bucketName: bucket, key: key })
       })
     })
 
@@ -328,7 +355,7 @@ describe('S3PersistorManagerTests', function() {
         S3ReadStream.on = sinon.stub()
         S3ReadStream.on.withArgs('error').yields(genericError)
         try {
-          stream = await S3PersistorManager.promises.getFileStream(bucket, key)
+          stream = await S3Persistor.promises.getFileStream(bucket, key)
         } catch (err) {
           error = err
         }
@@ -342,12 +369,12 @@ describe('S3PersistorManagerTests', function() {
         expect(error).to.be.an.instanceOf(Errors.ReadError)
       })
 
-      it('wraps the error from S3', function() {
-        expect(error.cause).to.equal(genericError)
+      it('wraps the error', function() {
+        expect(error.cause).to.exist
       })
 
       it('stores the bucket and key in the error', function() {
-        expect(error.info).to.deep.equal({ Bucket: bucket, Key: key })
+        expect(error.info).to.include({ bucketName: bucket, key: key })
       })
     })
   })
@@ -357,7 +384,7 @@ describe('S3PersistorManagerTests', function() {
       let size
 
       beforeEach(async function() {
-        size = await S3PersistorManager.promises.getFileSize(bucket, key)
+        size = await S3Persistor.promises.getFileSize(bucket, key)
       })
 
       it('should return the object size', function() {
@@ -380,7 +407,7 @@ describe('S3PersistorManagerTests', function() {
           promise: sinon.stub().rejects(S3NotFoundError)
         })
         try {
-          await S3PersistorManager.promises.getFileSize(bucket, key)
+          await S3Persistor.promises.getFileSize(bucket, key)
         } catch (err) {
           error = err
         }
@@ -403,7 +430,7 @@ describe('S3PersistorManagerTests', function() {
           promise: sinon.stub().rejects(genericError)
         })
         try {
-          await S3PersistorManager.promises.getFileSize(bucket, key)
+          await S3Persistor.promises.getFileSize(bucket, key)
         } catch (err) {
           error = err
         }
@@ -422,24 +449,62 @@ describe('S3PersistorManagerTests', function() {
   describe('sendStream', function() {
     describe('with valid parameters', function() {
       beforeEach(async function() {
-        return S3PersistorManager.promises.sendStream(bucket, key, ReadStream)
+        return S3Persistor.promises.sendStream(bucket, key, ReadStream)
       })
 
       it('should upload the stream', function() {
         expect(S3Client.upload).to.have.been.calledWith({
           Bucket: bucket,
           Key: key,
-          Body: 'readStream'
+          Body: MeteredStream
         })
       })
 
       it('should meter the stream', function() {
-        expect(ReadStream.pipe).to.have.been.calledWith(MeteredStream)
+        expect(Stream.pipeline).to.have.been.calledWith(
+          ReadStream,
+          MeteredStream
+        )
       })
 
       it('should record an egress metric', function() {
         expect(Metrics.count).to.have.been.calledWith('s3.egress', objectSize)
       })
+
+      it('calculates the md5 hash of the file', function() {
+        expect(Stream.pipeline).to.have.been.calledWith(ReadStream, Hash)
+      })
+    })
+
+    describe('when a hash is supploed', function() {
+      beforeEach(async function() {
+        return S3Persistor.promises.sendStream(
+          bucket,
+          key,
+          ReadStream,
+          'aaaaaaaabbbbbbbbaaaaaaaabbbbbbbb'
+        )
+      })
+
+      it('should not calculate the md5 hash of the file', function() {
+        expect(Stream.pipeline).not.to.have.been.calledWith(
+          sinon.match.any,
+          Hash
+        )
+      })
+
+      it('sends the hash in base64', function() {
+        expect(S3Client.upload).to.have.been.calledWith({
+          Bucket: bucket,
+          Key: key,
+          Body: MeteredStream,
+          ContentMD5: 'qqqqqru7u7uqqqqqu7u7uw=='
+        })
+      })
+
+      it('does not fetch the md5 hash of the uploaded file', function() {
+        expect(S3Client.headObject).not.to.have.been.called
+      })
     })
 
     describe('when the upload fails', function() {
@@ -449,7 +514,7 @@ describe('S3PersistorManagerTests', function() {
           promise: sinon.stub().rejects(genericError)
         })
         try {
-          await S3PersistorManager.promises.sendStream(bucket, key, ReadStream)
+          await S3Persistor.promises.sendStream(bucket, key, ReadStream)
         } catch (err) {
           error = err
         }
@@ -464,7 +529,7 @@ describe('S3PersistorManagerTests', function() {
   describe('sendFile', function() {
     describe('with valid parameters', function() {
       beforeEach(async function() {
-        return S3PersistorManager.promises.sendFile(bucket, key, filename)
+        return S3Persistor.promises.sendFile(bucket, key, filename)
       })
 
       it('should create a read stream for the file', function() {
@@ -475,7 +540,7 @@ describe('S3PersistorManagerTests', function() {
         expect(S3Client.upload).to.have.been.calledWith({
           Bucket: bucket,
           Key: key,
-          Body: 'readStream'
+          Body: MeteredStream
         })
       })
     })
@@ -486,7 +551,7 @@ describe('S3PersistorManagerTests', function() {
       beforeEach(async function() {
         Fs.createReadStream = sinon.stub().throws(FileNotFoundError)
         try {
-          await S3PersistorManager.promises.sendFile(bucket, key, filename)
+          await S3Persistor.promises.sendFile(bucket, key, filename)
         } catch (err) {
           error = err
         }
@@ -507,7 +572,7 @@ describe('S3PersistorManagerTests', function() {
       beforeEach(async function() {
         Fs.createReadStream = sinon.stub().throws(genericError)
         try {
-          await S3PersistorManager.promises.sendFile(bucket, key, filename)
+          await S3Persistor.promises.sendFile(bucket, key, filename)
         } catch (err) {
           error = err
         }
@@ -526,7 +591,7 @@ describe('S3PersistorManagerTests', function() {
   describe('copyFile', function() {
     describe('with valid parameters', function() {
       beforeEach(async function() {
-        return S3PersistorManager.promises.copyFile(bucket, key, destKey)
+        return S3Persistor.promises.copyFile(bucket, key, destKey)
       })
 
       it('should copy the object', function() {
@@ -546,7 +611,7 @@ describe('S3PersistorManagerTests', function() {
           promise: sinon.stub().rejects(S3NotFoundError)
         })
         try {
-          await S3PersistorManager.promises.copyFile(bucket, key, destKey)
+          await S3Persistor.promises.copyFile(bucket, key, destKey)
         } catch (err) {
           error = err
         }
@@ -561,7 +626,7 @@ describe('S3PersistorManagerTests', function() {
   describe('deleteFile', function() {
     describe('with valid parameters', function() {
       beforeEach(async function() {
-        return S3PersistorManager.promises.deleteFile(bucket, key)
+        return S3Persistor.promises.deleteFile(bucket, key)
       })
 
       it('should delete the object', function() {
@@ -580,7 +645,7 @@ describe('S3PersistorManagerTests', function() {
           promise: sinon.stub().rejects(S3NotFoundError)
         })
         try {
-          await S3PersistorManager.promises.deleteFile(bucket, key)
+          await S3Persistor.promises.deleteFile(bucket, key)
         } catch (err) {
           error = err
         }
@@ -595,7 +660,7 @@ describe('S3PersistorManagerTests', function() {
   describe('deleteDirectory', function() {
     describe('with valid parameters', function() {
       beforeEach(async function() {
-        return S3PersistorManager.promises.deleteDirectory(bucket, key)
+        return S3Persistor.promises.deleteDirectory(bucket, key)
       })
 
       it('should list the objects in the directory', function() {
@@ -621,7 +686,7 @@ describe('S3PersistorManagerTests', function() {
         S3Client.listObjects = sinon
           .stub()
           .returns({ promise: sinon.stub().resolves({ Contents: [] }) })
-        return S3PersistorManager.promises.deleteDirectory(bucket, key)
+        return S3Persistor.promises.deleteDirectory(bucket, key)
       })
 
       it('should list the objects in the directory', function() {
@@ -644,7 +709,7 @@ describe('S3PersistorManagerTests', function() {
           .stub()
           .returns({ promise: sinon.stub().rejects(genericError) })
         try {
-          await S3PersistorManager.promises.deleteDirectory(bucket, key)
+          await S3Persistor.promises.deleteDirectory(bucket, key)
         } catch (err) {
           error = err
         }
@@ -671,7 +736,7 @@ describe('S3PersistorManagerTests', function() {
           .stub()
           .returns({ promise: sinon.stub().rejects(genericError) })
         try {
-          await S3PersistorManager.promises.deleteDirectory(bucket, key)
+          await S3Persistor.promises.deleteDirectory(bucket, key)
         } catch (err) {
           error = err
         }
@@ -692,7 +757,7 @@ describe('S3PersistorManagerTests', function() {
       let size
 
       beforeEach(async function() {
-        size = await S3PersistorManager.promises.directorySize(bucket, key)
+        size = await S3Persistor.promises.directorySize(bucket, key)
       })
 
       it('should list the objects in the directory', function() {
@@ -714,7 +779,7 @@ describe('S3PersistorManagerTests', function() {
         S3Client.listObjects = sinon
           .stub()
           .returns({ promise: sinon.stub().resolves({ Contents: [] }) })
-        size = await S3PersistorManager.promises.directorySize(bucket, key)
+        size = await S3Persistor.promises.directorySize(bucket, key)
       })
 
       it('should list the objects in the directory', function() {
@@ -737,7 +802,7 @@ describe('S3PersistorManagerTests', function() {
           .stub()
           .returns({ promise: sinon.stub().rejects(genericError) })
         try {
-          await S3PersistorManager.promises.directorySize(bucket, key)
+          await S3Persistor.promises.directorySize(bucket, key)
         } catch (err) {
           error = err
         }
@@ -758,10 +823,7 @@ describe('S3PersistorManagerTests', function() {
       let exists
 
       beforeEach(async function() {
-        exists = await S3PersistorManager.promises.checkIfFileExists(
-          bucket,
-          key
-        )
+        exists = await S3Persistor.promises.checkIfFileExists(bucket, key)
       })
 
       it('should get the object header', function() {
@@ -783,10 +845,7 @@ describe('S3PersistorManagerTests', function() {
         S3Client.headObject = sinon
           .stub()
           .returns({ promise: sinon.stub().rejects(S3NotFoundError) })
-        exists = await S3PersistorManager.promises.checkIfFileExists(
-          bucket,
-          key
-        )
+        exists = await S3Persistor.promises.checkIfFileExists(bucket, key)
       })
 
       it('should get the object header', function() {
@@ -809,7 +868,7 @@ describe('S3PersistorManagerTests', function() {
           .stub()
           .returns({ promise: sinon.stub().rejects(genericError) })
         try {
-          await S3PersistorManager.promises.checkIfFileExists(bucket, key)
+          await S3Persistor.promises.checkIfFileExists(bucket, key)
         } catch (err) {
           error = err
         }