Re: [PATCH v3 2/8] maintenance: add loose-objects task

On 9/22/2020 7:09 PM, Jonathan Tan wrote:

quoted

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. 

[snip]

quoted

This has potential of
   happening in partial clones if someone runs 'git grep' or
   otherwise evades the batch-download feature for requesting
   promisor objects.

This part is not strictly true, as even when Git lazy-fetches one
object, it fetches it in the form of a packfile - so maybe remove this
sentence.

This is a good point. I just did some testing and we do store these
single-object downloads as pack-files. My misunderstanding is due to
my own bias and experience with the GVFS protocol.

I have also heard that "git fetch" might explode some small pack-files
into loose objects, and I guess I expected the same here. However, that
is not the case for partial clone. I'll remove this.

This is nevertheless a good feature to have - loose objects may not be
created during lazy fetches, but they definitely are created during
normal operation (e.g. commits). Git, as a whole, prefers packfiles over
loose objects, and just packing the loose objects themselves instead of
running repack (which goes through all reachable objects) is definitely
better for large repositories.

Thanks,
-Stolee