Re: Git packs friendly to block-level deduplication

From: Elijah Newren <newren@gmail.com>
Date: Jan 24, 2018, 22:37 UTC
Message-ID: <CABPp-BE0u9x_TtEHmfS11ZV-50rSvCi_y7cmTVV7z=2zT3atvg@mail.gmail.com>
In-Reply-To: <87bmhiykvw.fsf@evledraar.gmail.com>

On Wed, Jan 24, 2018 at 2:03 PM, Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:

Show 11 quoted lines

> If you have a bunch of git repositories cloned of the same project on
> the same filesystem, it would be nice of the packs that are produced
> would be friendly to block-level deduplication.
>
> This would save space, and the blocks would be more likely to be in
> cache when you access them, likely speeding up git operations even if
> the packing itself is less efficient.
>
> Here's a hacky one-liner that clones git/git and peff/git (almost the
> same content) and md5sums each 4k packed block, and sort | uniq -c's
> them to see how many are the same:

<snip>

>
> Has anyone here barked up this tree before? Suggestions? Tips on where
> to start hacking the repack code to accomplish this would be most
> welcome.

Does this overlap with the desire to have resumable clones? I'm curious what would happen if you did the same experiment with two separate clones of git/git, cloned one right after the other so that hopefully the upstream git/git didn't receive any updates between your two separate clones. (In other words, how much do packfiles differ in practice for different packings of the same data?)