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From: Stefan Beller <sbeller@google.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Jonathan Nieder <jrnieder@gmail.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Brandon Williams <bmwill@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Git Mailing List <git@vger.kernel.org>,
	Jonathan Tan <jonathantanmy@google.com>,
	Jeff King <peff@peff.net>, David Lang <david@lang.hm>,
	"brian m. carlson" <sandals@crustytoothpaste.net>
Subject: Re: RFC v3: Another proposed hash function transition plan
Date: Wed, 13 Sep 2017 15:07:08 -0700
Message-ID: <CAGZ79kakGcMJ7HuH+MPsMrvw40uGchr6H-SQw9-p8pgi3Yk_Bw@mail.gmail.com> (raw)
In-Reply-To: <xmqq7ex21d2v.fsf@gitster.mtv.corp.google.com>

On Wed, Sep 13, 2017 at 2:52 PM, Junio C Hamano <gitster@pobox.com> wrote:
> Jonathan Nieder <jrnieder@gmail.com> writes:
>
>> Treating generation numbers as derived data (as in Jeff King's
>> preferred design, if I have understood his replies correctly) would
>> also be possible but it does not interact well with shallow clone or
>> narrow clone.
>
> Just like we have skewed committer timestamps, there is no reason to
> believe that generation numbers embedded in objects are trustable,
> and there is no way for narrow clients to even verify their correctness.
>
> So I agree with Peff that having generation numbers in object is
> pointless; I agree any other derivables like corresponding sha-1
> name is also pointless to have.
>
> This is a tangent, but it may be fine for a shallow clone to treat
> the cut-off points in the history as if they are root commits and
> compute generation numbers locally, just like everybody else does.
> As generation numbers won't have to be global (because we will not
> be embedding them in objects), nobody gets hurt if they do not match
> across repositories---just like often-mentioned rename detection
> cache, it can be kept as a mere local performance aid and does not
> have to participate in the object model.

Locally it helps for some operations such as correct walks.
For the network case however, it doesn't really help either.

If we had global generation numbers, one could imagine that they
are used in the pack negotiation (server advertises the maximum
generation number or even gen number per branch; client
could binary search in there for the fork point)

I wonder if locally generated generation numbers (for the shallow
case) could be used somehow to still improve network operations.



>> My assumption based on previous conversations (and other external
>> conversations like [1]) is that we are going to use SHA2-256 and have
>> a pretty strong consensus for that.  Don't worry!
>
> Hmph, I actually re-read the thread recently, and my impression was
> that we didn't quite have a consensus but were leaning towards
> SHA3-256.
>
> I do not personally have a strong preference myself and I would say
> that anything will do as long as it is with good longevity and
> availability.  SHA2 family would be a fine choice due to its age on
> both counts, being scrutinized longer and having a chance to be
> implemented in many places, even though its age itself may have to
> be subtracted from the longevity factor.

If we'd get the transition somewhat right, the next transition will
be easier than the current transition, such that I am not that concerned
about longevity. I am rather concerned about the complexity that is added
to the code base (whilst accumulating technical debt instead of clearer
abstraction layers)

  reply index

Thread overview: 113+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-04  1:12 RFC: " Jonathan Nieder
2017-03-05  2:35 ` Linus Torvalds
2017-03-06  0:26   ` brian m. carlson
2017-03-06 18:24     ` Brandon Williams
2017-06-15 10:30       ` Which hash function to use, was " Johannes Schindelin
2017-06-15 11:05         ` Mike Hommey
2017-06-15 13:01           ` Jeff King
2017-06-15 16:30             ` Ævar Arnfjörð Bjarmason
2017-06-15 19:34               ` Johannes Schindelin
2017-06-15 21:59                 ` Adam Langley
2017-06-15 22:41                   ` brian m. carlson
2017-06-15 23:36                     ` Ævar Arnfjörð Bjarmason
2017-06-16  0:17                       ` brian m. carlson
2017-06-16  6:25                         ` Ævar Arnfjörð Bjarmason
2017-06-16 13:24                           ` Johannes Schindelin
2017-06-16 17:38                             ` Adam Langley
2017-06-16 20:52                               ` Junio C Hamano
2017-06-16 21:12                                 ` Junio C Hamano
2017-06-16 21:24                                   ` Jonathan Nieder
2017-06-16 21:39                                     ` Ævar Arnfjörð Bjarmason
2017-06-16 20:42                             ` Jeff King
2017-06-19  9:26                               ` Johannes Schindelin
2017-06-15 21:10             ` Mike Hommey
2017-06-16  4:30               ` Jeff King
2017-06-15 17:36         ` Brandon Williams
2017-06-15 19:20           ` Junio C Hamano
2017-06-15 19:13         ` Jonathan Nieder
2017-03-07  0:17   ` RFC v3: " Jonathan Nieder
2017-03-09 19:14     ` Shawn Pearce
2017-03-09 20:24       ` Jonathan Nieder
2017-03-10 19:38         ` Jeff King
2017-03-10 19:55           ` Jonathan Nieder
2017-09-28  4:43       ` [PATCH v4] technical doc: add a design doc for hash function transition Jonathan Nieder
2017-09-29  6:06         ` Junio C Hamano
2017-09-29  8:09           ` Junio C Hamano
2017-09-29 17:34           ` Jonathan Nieder
2017-10-02  8:25             ` Junio C Hamano
2017-10-02 19:41             ` Jason Cooper
2017-10-02  9:02         ` Junio C Hamano
2017-10-02 19:23         ` Jason Cooper
2017-10-03  5:40         ` Junio C Hamano
2017-10-03 13:08           ` Jason Cooper
2017-10-04  1:44         ` Junio C Hamano
2017-09-06  6:28     ` RFC v3: Another proposed hash function transition plan Junio C Hamano
2017-09-08  2:40       ` Junio C Hamano
2017-09-08  3:34         ` Jeff King
2017-09-11 18:59         ` Brandon Williams
2017-09-13 12:05           ` Johannes Schindelin
2017-09-13 13:43             ` demerphq
2017-09-13 22:51               ` Jonathan Nieder
2017-09-14 18:26                 ` Johannes Schindelin
2017-09-14 18:40                   ` Jonathan Nieder
2017-09-14 22:09                     ` Johannes Schindelin
2017-09-13 23:30               ` Linus Torvalds
2017-09-14 18:45                 ` Johannes Schindelin
2017-09-18 12:17                   ` Gilles Van Assche
2017-09-18 22:16                     ` Johannes Schindelin
2017-09-19 16:45                       ` Gilles Van Assche
2017-09-29 13:17                         ` Johannes Schindelin
2017-09-29 14:54                           ` Joan Daemen
2017-09-29 22:33                             ` Johannes Schindelin
2017-09-30 22:02                               ` Joan Daemen
2017-10-02 14:26                                 ` Johannes Schindelin
2017-09-18 22:25                     ` Jonathan Nieder
2017-09-26 17:05                   ` Jason Cooper
2017-09-26 22:11                     ` Johannes Schindelin
2017-09-26 22:25                       ` [PATCH] technical doc: add a design doc for hash function transition Stefan Beller
2017-09-26 23:38                         ` Jonathan Nieder
2017-09-26 23:51                       ` RFC v3: Another proposed hash function transition plan Jonathan Nieder
2017-10-02 14:54                         ` Jason Cooper
2017-10-02 16:50                           ` Brandon Williams
2017-10-02 14:00                       ` Jason Cooper
2017-10-02 17:18                         ` Linus Torvalds
2017-10-02 19:37                           ` Jeff King
2017-09-13 16:30             ` Jonathan Nieder
2017-09-13 21:52               ` Junio C Hamano
2017-09-13 22:07                 ` Stefan Beller [this message]
2017-09-13 22:18                   ` Jonathan Nieder
2017-09-14  2:13                     ` Junio C Hamano
2017-09-14 15:23                       ` Johannes Schindelin
2017-09-14 15:45                         ` demerphq
2017-09-14 22:06                           ` Johannes Schindelin
2017-09-13 22:15                 ` Junio C Hamano
2017-09-13 22:27                   ` Jonathan Nieder
2017-09-14  2:10                     ` Junio C Hamano
2017-09-14 12:39               ` Johannes Schindelin
2017-09-14 16:36                 ` Brandon Williams
2017-09-14 18:49                 ` Jonathan Nieder
2017-09-15 20:42                   ` Philip Oakley
2017-03-05 11:02 ` RFC: " David Lang
     [not found]   ` <CA+dhYEXHbQfJ6KUB1tWS9u1MLEOJL81fTYkbxu4XO-i+379LPw@mail.gmail.com>
2017-03-06  9:43     ` Jeff King
2017-03-06 23:40   ` Jonathan Nieder
2017-03-07  0:03     ` Mike Hommey
2017-03-06  8:43 ` Jeff King
2017-03-06 18:39   ` Jonathan Tan
2017-03-06 19:22     ` Linus Torvalds
2017-03-06 19:59       ` Brandon Williams
2017-03-06 21:53       ` Junio C Hamano
2017-03-07  8:59     ` Jeff King
2017-03-06 18:43   ` Junio C Hamano
2017-03-07 18:57 ` Ian Jackson
2017-03-07 19:15   ` Linus Torvalds
2017-03-08 11:20     ` Ian Jackson
2017-03-08 15:37       ` Johannes Schindelin
2017-03-08 15:40       ` Johannes Schindelin
2017-03-20  5:21         ` Use base32? Jason Hennessey
2017-03-20  5:58           ` Michael Steuer
2017-03-20  8:05             ` Jacob Keller
2017-03-21  3:07               ` Michael Steuer
2017-03-13  9:24 ` RFC: Another proposed hash function transition plan The Keccak Team
2017-03-13 17:48   ` Jonathan Nieder
2017-03-13 18:34     ` ankostis
2017-03-17 11:07       ` Johannes Schindelin

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