Zuul mergers create a vast number of git references under /refs/zuul
which are never garbage collected.
With hundred of thousands of references, that makes git fetch operations
very slow since git uploads all references to Gerrit to synchronize the
Zuul maintained repository. On one of Wikimedia busy repository
(mediawiki/core) we had 55000 such references and it can take up to 18
seconds for a fetch to complete. I have seen occurences of a merge
taking 2 minutes to complete.
As such, this tiny script clears out references for which the commit date
of the pointed commit object is older than 360 days (the default).
It is not perfect since a recent reference can well point to an old
object. That would be the case on repositories that are barely active.
In such case the ref will be gone despite it being recently created.
A better way would be to vary Zuul references by using month/day which
will let one easily garbage collect them. But I am being lazy and that
would not let us clear out references using the current scheme.
zuul-clear-refs.py --verbose --dry-run --until 90 /srv/zuul/git/project
Would show a list of references pointing to commit dates older than 90
days and output a message whenever the script would delete them.
Hint about the utility in our merger documentation.