One goal of background maintenance jobs is to allow a user to
disable auto-gc (gc.auto=0) but keep their repository in a clean
state. Without any cleanup, loose objects will clutter the object
database and slow operations. In addition, the loose objects will
take up extra space because they are not stored with deltas against
similar objects.
Create a 'loose-objects' task for the 'git maintenance run' command.
This helps clean up loose objects without disrupting concurrent Git
commands using the following sequence of events:
1. Run 'git prune-packed' to delete any loose objects that exist
in a pack-file. Concurrent commands will prefer the packed
version of the object to the loose version. (Of course, there
are exceptions for commands that specifically care about the
location of an object. These are rare for a user to run on
purpose, and we hope a user that has selected background
maintenance will not be trying to do foreground maintenance.)
2. Run 'git pack-objects' on a batch of loose objects. These
objects are grouped by scanning the loose object directories in
lexicographic order until listing all loose objects -or-
reaching 50,000 objects. This is more than enough if the loose
objects are created only by a user doing normal development.
We noticed users with _millions_ of loose objects because VFS
for Git downloads blobs on-demand when a file read operation
requires populating a virtual file.
This step is based on a similar step in Scalar [1] and VFS for Git.
[1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/LooseObjectsStep.cs
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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