We had a CI outage caused by full SSD disks on gallium and lanthanum. The reason is apparently that l10n bot triggered testextensions jobs on almost all extensions, that caused mediawiki/core to be cloned by zuul-cloner from Gerrit multiple times and filled the disks quite fast.
MediaWiki core takes roughly 500MB. zuul-cloner does a regular git clone.
When we used the Jenkins git plugin, we had it set to use git clone --reference /some/mirror which made it way faster to realize the clone. zuul-cloner does not do such a thing.
We could have a copy of mediawiki/core on the slaves (and on the same disk as the workspace), then before invoking zuul-cloner on mediawiki/core, do a regular git clone which would creates hardlinks and save disk. zuul-cloner would then fetch whatever is missing from zuul git repositories.