Each project in Gerrit is stored in a bare Git repository. Gerrit uses
the JGit library to access (read and write to) these Git repositories.
As modifications are made to a project, Git repository maintenance will
be needed or performance will eventually suffer. When using the Git
command line tool to operate on a Git repository, it will run
every now and then on the repository to ensure that Git garbage
collection is performed. However regular maintenance does not happen as
a result of normal Gerrit operations, so this is something that Gerrit
administrators need to plan for.
Gerrit has a built-in feature which allows it to run Git garbage
collection on repositories. This can be
configured to run on a regular basis, and/or
this can be run manually with the gerrit gc ssh
command, or with the run-gc REST API.
Some administrators will opt to run
git gc or
jgit gc outside of
Gerrit instead. There are many reasons this might be done, the main one
likely being that when it is run in Gerrit it can be very resource
intensive and scheduling an external job to run Git garbage collection
allows administrators to finely tune the approach and resource usage of
Git Garbage Collection Impacts
Unlike a typical server database, access to Git repositories is not marshalled through a single process or a set of inter communicating processes. Unfortuntatlely the design of the on-disk layout of a Git repository does not allow for 100% race free operations when accessed by multiple actors concurrently. These design shortcomings are more likely to impact the operations of busy repositories since racy conditions are more likely to occur when there are more concurrent operations. Since most Gerrit servers are expected to run without interruptions, Git garbage collection likely needs to be run during normal operational hours. When it runs, it adds to the concurrency of the overall accesses. Given that many of the operations in garbage collection involve deleting files and directories, it has a higher chance of impacting other ongoing operations than most other operations.
When Git garbage collection deletes a file or directory that is currently in use by an ongoing operation, it can cause that operation to fail. These sorts of failures are often single shot failures, i.e. the operation will succeed if tried again. An example of such a failure is when a pack file is deleted while Gerrit is sending an object in the file over the network to a user performing a clone or fetch. Usually pack files are only deleted when the referenced objects in them have been repacked and thus copied to a new pack file. So performing the same operation again after the fetch will likely send the same object from the new pack instead of the deleted one, and the operation will succeed.
It is possible for data loss to occur when Git garbage collection runs. This is very rare, but it can happen. This can happen when an object is believed to be unreferenced when object repacking is running, and then garbage collection deletes it. This can happen because even though an object may indeed be unreferenced when object repacking begins and reachability of all objects is determined, it can become referenced by another concurrent operation after this unreferenced determination but before it gets deleted. When this happens, a new reference can be created which points to a now missing object, and this will result in a loss.
Reducing Git Garbage Collection Impacts
JGit has a
preserved directory feature which is intended to reduce
some of the impacts of Git garbage collection, and Gerrit can take
advantage of the feature too. The
preserved directory is a
subdirectory of a repository’s
objects/pack directory where JGit will
move pack files that it would normally delete when
jgit gc is invoked
--preserve-oldpacks option. It will later delete these files
the next time that
jgit gc is run if it is invoked with the
--prune-preserved option. Using these flags together on every
invocation means that packfiles will get an extended lifetime by one
full garbage collection cycle. Since an atomic move is used to move these
files, any open references to them will continue to work, even on NFS. On
a busy repository, preserving pack files can make operations much more
reliable, and interrupted operations should almost entirely disappear.
Moving files to the
preserved directory also has the ability to reduce
data loss. If JGit cannot find an object it needs in its current object
DB, it will look into the
preserved directory as a last resort. If it
finds the object in a pack file there, it will restore the
slated-to-be-deleted pack file back to the original
directory effectively "undeleting" it and making all the objects in it
available again. When this happens, data loss is prevented.
One advantage of restoring preserved packfiles in this way when an
object is referenced in them, is that it makes loosening unreferenced
objects during Git garbage collection, which is a potentially expensive,
wasteful, and performance impacting operation, no longer desirable. It
is recommended that if you use Git for garbage collection, that you use
-a option to
git repack instead of the
-A option to no longer
perform this loosening.
When Git is used for garbage collection instead of JGit, it is fairly
easy to wrap
git gc or
git repack with a small script which has a
--prune-preserved option which behaves as mentioned above by deleting
any pack files currently in the preserved directory, and also has a
--preserve-oldpacks option which then hardlinks all the currently
existing pack files from the
objects/pack directory into the
preserved directory right before calling the real Git command. This
approach will then behave similarly to
jgit gc with respect to
preserving pack files.
Part of Gerrit Code Review