From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Elijah Newren <newren@gmail.com>
Cc: Git mailing list <git@vger.kernel.org>,
Junio C Hamano <gitster@pobox.com>, Jeff King <peff@peff.net>
Subject: Re: Git packs friendly to block-level deduplication
Date: Thu, 25 Jan 2018 00:06:59 +0100 [thread overview]
Message-ID: <878tcmyhyk.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <CABPp-BE0u9x_TtEHmfS11ZV-50rSvCi_y7cmTVV7z=2zT3atvg@mail.gmail.com>
On Wed, Jan 24 2018, Elijah Newren jotted:
> On Wed, Jan 24, 2018 at 2:03 PM, Ævar Arnfjörð Bjarmason
> <avarab@gmail.com> wrote:
>> 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?)
If you clone git/git from Github twice in a row you get the exact same
pack, and AFAICT this is true of git in general (but may change between
versions).
If you make a local commit to that, copy the dir, and repack -A -d you
get the exact same packs again.
If you then make just one local commit to one copy (even with
--allow-empty) and repack, you get entirely differnt packs, in my test
2.5% of the blocks remain the same.
Obviously you could pack *that* new content incrementally and keep the
existing pack, but that won't help you with de-duping the initially
cloned data, which is what matters.
next prev parent reply other threads:[~2018-01-24 23:07 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-24 22:03 Git packs friendly to block-level deduplication Ævar Arnfjörð Bjarmason
2018-01-24 22:19 ` Mike Hommey
2018-01-24 22:23 ` Junio C Hamano
2018-01-24 22:30 ` Mike Hommey
2018-01-24 22:47 ` Ævar Arnfjörð Bjarmason
2018-01-24 22:25 ` Eric Wong
2018-01-24 22:37 ` Elijah Newren
2018-01-24 23:06 ` Ævar Arnfjörð Bjarmason [this message]
2018-01-24 23:32 ` Jeff King
2018-01-24 23:22 ` Jeff King
2018-01-25 0:03 ` Ævar Arnfjörð Bjarmason
2018-01-25 0:10 ` Jeff King
2018-01-25 0:29 ` Jeff King
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