linux-kernel - Re: Minimum git commit abbrev length (Was Re: -tip: origin tree build failure (was: [GIT PULL] ext4 update) for 2.6.37)

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Message-ID: <AANLkTi=8tq1zu_RWjmk5rLOWvP_KPH5UQioQ6wEkc0Ob@mail.gmail.com>
Date:	Thu, 28 Oct 2010 11:28:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Ted Ts'o" <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	git@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Minimum git commit abbrev length (Was Re: -tip: origin tree build
 failure (was: [GIT PULL] ext4 update) for 2.6.37)

On Thu, Oct 28, 2010 at 10:27 AM, Ted Ts'o <tytso@....edu> wrote:
> On Thu, Oct 28, 2010 at 07:17:01PM +0200, Ingo Molnar wrote:
>> > Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> > Yes. Except for the kernel the default git commit abbreviation is
>> > borderline too short. Seven hex-chars can easily alias with a few
>> > more pulls from me: git will not give aliases at the time it gives
>> > a shorthand, but a month or two later the abbreviated commit may
>> > no longer be unique.
>> >
>> > So I suggest using --abbrev=12 or similar.
>>
>> ok. A helper script i use does this:
>>
>>    git log --pretty=format:"%h: %s" $@
>>
>> I have added --abbrev=12. Might make sense to lengthen the %h
>> default in upstream Git as well?
>
> Maybe the right thing to do is add a git config option which allows
> for a configurable minimum git commit abbreviation length?

Yes. The default of 7 (I think) comes from fairly early in git
development, when seven hex digits was a lot (it covers about 250+
million hash values). Back then I thought that 65k revisions was a lot
(it was what we were about to hit in BK), and each revision tends to
be about 5-10 new objects or so, so a million objects was a big
number.

These days, the kernel isn't even the largest git project, and even
the kernel has about 220k revisions (_much_ bigger than the BK tree
ever was) and we are approaching two million objects. At that point,
seven hex digits is still unique for a lot of them, but when we're
talking about just two orders of magnitude difference between number
of objects and the hash size, there _will_ be hash collisions. It's no
longer even close to unrealistic - it happens all the time.

So I suspect we should both increase the default abbrev that was
unrealistically small, _and_ add a way for people to set their own
default per-project in the git config file.

Maybe something like the attached (not necessarily well-thought-out or
well-tested: I also didn't actually change the default, although I
suspect we should up it from 7 to at least 10).

                        Linus

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