Docker Image Cleaner

A Python package (docker-image-cleaner) and associated Docker image (quay.io/jupyterhub/docker-image-cleaner) to clean up old docker images when a disk is running low on inodes or space.

The script has initially been developed to help installations of BinderHub clean up space on nodes as it otherwise can run out of space and stop being able to build now docker images.

Why?

Container images are one of the biggest consumers of disk space and inodes on kubernetes nodes. Kubernetes tries to make sure there is enough disk space on each node by garbage collecting unused container images and containers. Tuning this is important for binderhub installations, as many images are built and used only a couple times. However, on most managed kubernetes installations (like GKE, EKS, etc), we can not tune these parameters!

This script approximates the specific parts of the kubernetes container image garbage collection in a configurable way.

Requirements

Only kubernetes nodes using the docker runtime are supported. containerd or cri-o container backends are not supported.
The script expects to run in a kubernetes DaemonSet, with /var/lib/docker from the node mounted inside the container. This lets the script figure out how much disk space docker container images are actually using.
The DaemonSet should have a ServiceAccount attached that has permissions to talk to the kubernetes API and cordon / uncordon nodes. This makes sure new pods are not scheduled on to the node while image cleaning is happening, as it can take a while.

How does it work?

Compute how much space /var/lib/docker directory (specified by the PATH_TO_CHECK environment variable) is taking up.
If the disk space used is greater than the garbage collection trigger threshold (specified by IMAGE_GC_THRESHOLD_HIGH), garbage collection is triggered. If not, the script just waits another 5 minutes (set by IMAGE_GC_INTERVAL).
If garbage collection is triggered, the kubernetes node is first cordoned to prevent any new pods from being scheduled on it for the duration of the garbage collection.
Unused container images are deleted one by one, starting with the biggest, until the disk space used by /var/lib/docker falls below the garbage collection 'ok' threshold (specified by IMAGE_GC_THRESHOLD_LOW). This low / high system makes sure we don't get too aggressive in cleaning the disk, as images being present on the node does speed up binderhub launches.
After the garbage collection is done, the kubernetes node is also uncordoned.
When done, we wait another 5 minutes (set by IMAGE_GC_INTERVAL), and repeat the whole process.

Configuration options

Currently, environment variables are used to set configuration for now.

Env variable	Description	Default
`PATH_TO_CHECK`	Path to `/var/lib/docker` directory used by the docker daemon	`/var/lib/docker`
`IMAGE_GC_INTERVAL`	Amount of time (in seconds) to wait between checking if GC needs to be triggered	`300`
`IMAGE_GC_DELAY`	Amount of time (in seconds) to wait between deleting container images, so we don't DOS the docker API	`1`
`IMAGE_GC_THRESHOLD_TYPE`	Determine if GC should be triggered based on relative or absolute disk usage	`relative`
`IMAGE_GC_THRESHOLD_HIGH`	% or absolute disk space available (based on `IMAGE_GC_THRESHOLD_TYPE`) when we start deleting container images	`80`
`IMAGE_GC_THRESHOLD_LOW`	% or absolute disk space available (based on `IMAGE_GC_THRESHOLD_TYPE`) when we can stop deleting container images	`60`

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