On Thu, Jul 07, 2016 at 10:09:17PM +0300, Kirill Smelkov wrote:
> Starting from 6b8fda2d (pack-objects: use bitmaps when packing objects)
> if a repository has bitmap index, pack-objects can nicely speedup
> "Counting objects" graph traversal phase. That however was done only for
> case when resultant pack is sent to stdout, not written into a file.
>
> We can teach pack-objects to use bitmap index for initial object
> counting phase when generating resultant pack file too:
I'm not sure this is a good idea in general. When bitmaps are in use, we
cannot fill out the details in the object-packing list as thoroughly. In
particular:
- we will not compute the same write order (which is based on
traversal order), leading to packs that have less efficient cache
characteristics
- we don't learn about the filename of trees and blobs, which is going
to make the delta step much less efficient. This might be mitigated
by turning on the bitmap name-hash cache; I don't recall how much
detail pack-objects needs on the name (i.e., the full name versus
just the hash).
There may be other subtle things, too. The general idea of tying the
bitmap use to pack_to_stdout is that you _do_ want to use it for
serving fetches and pushes, but for a full on-disk repack via gc, it's
more important to generate a good pack.
Your use case:
> git-backup extracts many packs on repositories restoration. That was my
> initial motivation for the patch.
Seems to be somewhere in between. I'm not sure I understand how you're
invoking pack-objects here, but I wonder if you should be using
"pack-objects --stdout" yourself.
But even if it is the right thing for your use case to be using bitmaps
to generate an on-disk bitmap, I think we should be making sure it
_doesn't_ trigger when doing a normal repack.
-Peff
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