On Wed, Oct 30, 2013 at 9:10 AM, Jeff King <p...@peff.net> wrote:
> In fact, I'm not quite sure that even a partial reuse up to an offset is
> 100% safe. In a newly packed git repo it is, because we always put bases
> before deltas (and OFS_DELTA objects need this). But if you had a bitmap
> generated from a fixed thin pack, we would have REF_DELTA objects early
> on that depend on bases appended to the end of the pack. So I really
> wonder if we should scrap this partial reuse and either just have full
> reuse, or go through the regular object_entry construction.
> Vicent, you've thought about the reuse code a lot more than I have. Any
> thoughts?

Yes, our pack writing and bitmap code takes enough precautions to
arrange the objects in the packfile in a way that can be partially
reused, so for any given bitmap file written from Git, I'd say we're
safe to always reuse the leader of the pack if this is possible.

For bitmaps generated from JGit, however, we cannot make this
assumption. I mean, we can right now (from my understanding of the
current implementation for pack-objects on JGit), but they are free to
change this in the future.

Obviously I intend to keep the pack reuse on production because the
CPU savings are noticeable, but we can drop it from the public
patchset. Ideally, we'd have full pack reuse like JGit, but we cannot
reasonably do that in GitHub because splitting a pack for the network
root would double our disk usage for all the forks.

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