The Apptainer cache

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• Why does Apptainer use a local cache?

• Where does Apptainer store images?

Objectives

• Learn about Apptainer’s image cache.

• Learn how to manage Apptainer images stored locally.

Apptainer’s image cache

While Apptainer doesn’t have a local image repository in the same way as Docker, it does cache downloaded image files. As we saw in the previous episode, images are simply .sif files stored on your local disk.

If you delete a local .sif image that you have pulled from a remote image repository and then pull it again, if the image is unchanged from the version you previously pulled, you will be given a copy of the image file from your local cache rather than the image being downloaded again from the remote source. This removes unnecessary network transfers and is particularly useful for large images which may take some time to transfer over the network. To demonstrate this, remove the hello-world.sif file stored in your test directory and then issue the pull command again:

$ rm hello-world.sif
$ apptainer pull hello-world.sif shub://vsoch/hello-world

INFO:    Use image from cache

As we can see in the above output, the image has been returned from the cache and we don’t see the output that we saw previously showing the image being downloaded from Apptainer Hub.

How do we know what is stored in the local cache? We can find out using the apptainer cache command:

$ apptainer cache list

There are 1 container file(s) using 59.75 MiB and 0 oci blob file(s) using 0.00 KiB of space
Total space used: 59.75 MiB

This tells us how many container files are stored in the cache and how much disk space the cache is using but it doesn’t tell us what is actually being stored. To find out more information we can add the -v verbose flag to the list command:

$ apptainer cache list -v

NAME                     DATE CREATED           SIZE             TYPE
3bac21df631874e3cbb3f0   2024-04-17 15:22:43    59.75 MiB        shub

There are 1 container file(s) using 59.75 MiB and 0 oci blob file(s) using 0.00 KiB of space
Total space used: 59.75 MiB

This provides us with some more useful information about the actual images stored in the cache. In the TYPE column we can see that our image type is shub because it’s a SIF image that has been pulled from Apptainer Hub.

Cleaning the Apptainer image cache

We can remove images from the cache using the apptainer cache clean command. Running the command without any options will display a warning and ask you to confirm that you want to remove everything from your cache.

You can also remove specific images or all images of a particular type. Look at the output of apptainer cache clean --help for more information.

Cache location

By default, Apptainer uses $HOME/.apptainer/cache as the location for the cache. You can change the location of the cache by setting the APPTAINER_CACHEDIR environment variable to the cache location you want to use.

Key Points

• Apptainer caches downloaded images so that an unchanged image isn’t downloaded again when it is requested using the apptainer pull command.

• You can free up space in the cache by removing all locally cached images or by specifying individual images to remove.