The Singularity cache

Overview

Questions

• Why does Singularity use a local cache?
• Where does Singularity store images?

Objectives

• Learn about Singularity’s image cache.
• Learn how to manage Singularity images stored locally.

Singularity’s image cache

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While Singularity 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:

BASH

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

OUTPUT

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 Singularity Hub.

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

BASH

$ singularity cache list

OUTPUT

There are 1 container file(s) using 62.65 MB and 0 oci blob file(s) using 0.00 kB of space
Total space used: 62.65 MB

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:

BASH

$ singularity cache list -v

OUTPUT

NAME                     DATE CREATED           SIZE             TYPE
hello-world_latest.sif   2020-04-03 13:20:44    62.65 MB         shub

There are 1 container file(s) using 62.65 MB and 0 oci blob file(s) using 0.00 kB of space
Total space used: 62.65 MB

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 Singularity Hub.

Cleaning the Singularity image cache

We can remove images from the cache using the singularity 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 singularity cache clean --help for more information.

Cache location

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

Key Points

• Singularity caches downloaded images so that an unchanged image isn’t downloaded again when it is requested using the singularity pull command.
• You can free up space in the cache by removing all locally cached images or by specifying individual images to remove.