Re: [PATCH 0/2] optimizing pack access on "read only" fetch repos

Jeff King [off-list ref] writes:

This is a repost from here:

  http://thread.gmane.org/gmane.comp.version-control.git/211176

which got no response initially. Basically the issue is that read-only
repos (e.g., a CI server) whose workflow is something like:

  git fetch $some_branch &&
  git checkout -f $some_branch &&
  make test

will never run git-gc, and will accumulate a bunch of small packs and
loose objects, leading to poor performance.

Patch 1 runs "gc --auto" on fetch, which I think is sane to do.

Patch 2 optimizes our pack dir re-scanning for fetch-pack (which, unlike
the rest of git, should expect to be missing lots of objects, since we
are deciding what to fetch).

I think 1 is a no-brainer. If your repo is packed, patch 2 matters less,
but it still seems like a sensible optimization to me.

  [1/2]: fetch: run gc --auto after fetching
  [2/2]: fetch-pack: avoid repeatedly re-scanning pack directory

-Peff

Both makes sense to me.

I also wonder if we would be helped by another "repack" mode that
coalesces small packs into a single one with minimum overhead, and
run that often from "gc --auto", so that we do not end up having to
have 50 packfiles.

When we have 2 or more small and young packs, we could:

 - iterate over idx files for these packs to enumerate the objects
   to be packed, replacing read_object_list_from_stdin() step;

 - always choose to copy the data we have in these existing packs,
   instead of doing a full prepare_pack(); and

 - use the order the objects appear in the original packs, bypassing
   compute_write_order().

The procedure cannot be a straight byte-for-byte copy, because some
objects may appear in multiple packs, and extra copies of the same
object have to be excised from the result.  OFS_DELTA offsets need
to be adjusted for objects that appear later in the output and for
objects that were deltified against such an object that recorded its
base with OFS_DELTA format.

But other than such OFS_DELTA adjustments, it feels that such an
"only coalesce multiple packs into one" mode should be fairly quick.