I've already set up a daily cronjob that prunes out all cache file older than 90 days. That's based on mtime. I suppose atime would be the better measure, but /mnt/nfs/labstore-secondary-tools-project is mounted with noatime. Do you want me to reduce the expiration age to 30 days? Alternatively I could come up with a dabade approach to implement atime myself and purge images that are not being accessed for a while.

Those are both good ideas. If 30 days works and doesn't ruin the tool's usefulness, that would definitely help and seems easy. There's a lot of larger tools this round with no single huge user, so I'm trying to see what all of the largest users can do to help.