From 069bcd1b1974cac905d158d1007af9aa7a8073ce Mon Sep 17 00:00:00 2001
From: Brandon Rozek <rozekbrandon@gmail.com>
Date: Mon, 28 Sep 2020 23:49:33 -0400
Subject: [PATCH] New Post

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+---
+title: "Auto Purge Old Docker Images"
+date: 2020-09-28T23:30:22-04:00
+draft: false
+tags: []
+---
+
+I use [Watchtower](https://github.com/containrrr/watchtower) to automatically update the docker images I use. After leaving it for several months, I've realized that I have been storing over 100GB of old docker images. I needed a way to automatically purge  old images and [Systemd Timers](https://opensource.com/article/20/7/systemd-timers) is the solution.
+
+First it's useful to know the docker command that purges unused images that are older than 24 hours old.
+
+```bash
+docker image prune -fa --filter "until=24h"
+```
+
+Then we can create a oneshot service file that will describe its dependencies and descriptions for Systemd to manage. This file is `/etc/systemd/system/docker-purge.service`.
+
+```yml
+[Unit]
+Description=Purge Docker Images Older than 24 Hours
+Requires=docker.service
+Wants=docker-purge.timer
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/docker image prune -fa --filter "until=24h"
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Now we can create the systemd timer that will hook to this service. This is `/etc/systemd/system/docker-purge.timer`.
+
+```yaml
+[Unit]
+Description=Purge Docker Images Older than 24 Hours
+Requires=docker-purge.service
+
+[Timer]
+Unit=docker-purge.service
+OnCalendar=*-*-* 00:00:00
+AccuracySec=24h
+
+[Install]
+WantedBy=timers.target
+```
+
+This tells Systemd to run the service every day if enabled/started, though not necessarily at midnight. Systemd will schedule a time to run the service within the `AccuracySec` parameter. That is, it will schedule a time to run sometime everyday. 
+
+Finally, let's enable and start the timer.
+
+```bash
+sudo systemctl enable docker-purge
+sudo systemctl start docker-purge
+```
+
+We can check the time that docker-purge is scheduled to run next by asking Systemd to list its timers.
+
+```bash
+systemctl list-timers
+```
+
+If you want to play around with the `OnCalendar` parameter.  A useful command is
+
+```bash
+systmed-analyze calendar --iterations=$N "$TIME"
+```
+
+Where you replace `$N` with an integer, and `$TIME` with your sample time string.
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