From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Derrick Stolee <derrickstolee@github.com>
Cc: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
git@vger.kernel.org, me@ttaylorr.com, gitster@pobox.com,
abhishekkumar8222@gmail.com
Subject: Re: [PATCH 5/7] commit-graph: document file format v2
Date: Mon, 28 Feb 2022 22:14:16 +0100 [thread overview]
Message-ID: <220228.86ilsy3a8b.gmgdl@evledraar.gmail.com> (raw)
In-Reply-To: <e74d72bc-b6f5-2e1c-63d1-d3a580f3dc11@github.com>
On Mon, Feb 28 2022, Derrick Stolee wrote:
> On 2/28/2022 9:27 AM, Ævar Arnfjörð Bjarmason wrote:
>>
>> On Mon, Feb 28 2022, Derrick Stolee wrote:
>>
>>> On 2/25/2022 5:31 PM, Ævar Arnfjörð Bjarmason wrote:
>
>>>> Or maybe they won't. I just found it surprising when reviewing this to
>>>> not find an answer to why that approach wasn't
>>>> considered.
>>>
>>> The point is to create a new format that can be chosen when deployed
>>> in an environment where older Git versions will not exist (such as
>>> a Git server). The new version is not chosen by default and instead
>>> is opt-in through the commitGraph.generationVersion config option.
>>>
>>> Perhaps in a year or two we would consider making this the new
>>> default, but there is no rush to do so.
>>
>> Looking into this a bit more I think that in either case this is less of
>> a big deal after my 43d35618055 (commit-graph write: don't die if the
>> existing graph is corrupt, 2019-03-25), which came out of some of those
>> discussions at the time of [1].
>>
>> I.e. now a client that only understands version N-1 will warn when
>> loading it, wheras it's only if a pre-v2.22.0 client (which has that
>> commit) reads the repository that we'd hard die on it, correct?
>>
>> But speaking of hyper-focus. I think that arguably applies to you in
>> this case when considering the trade-offs of these sorts of format
>> changes :)
>>
>> I.e. you're primarily considering cases of say a git server (presumably
>> running on GitHub) or another such deployment where it's easy to have
>> full control over all of your versions "in the wild".
>
> I'm thinking of servers, yes, but also 99% of clients who only upgrade
> (or _maybe_ downgrade to a recent, previous version occasionally).
*nod*
>> And thus a three-phase rollout of something like a format change can be
>> done in a timely and predictable manner.
>>
>> But git is used by *a lot* of people in a bunch of different
>> scenarios. E.g.:
>>
>> * A shared (hopefully read-only) NFS mounted by remote "unmanaged" clients.
>> * A tarred-up directory including a .git, which may be transferred to
>> a machine with a pre-v2.22.0 version.
>>
>> Or even softer cases of failure, such as:
>>
>> * A cronjob causes an alert/incident somewhere because the server
>> operator started writing a new version, but forgot about a set
>> of machines that are still on the old version.
>
> It is important to continue supporting these cases, and this change does
> not cause any issues for them.
The issues in those cases will range from warnings on older versions
when loading the graph to errors if it's pre-v2.22.0, with the
performance benefits v3 placing them out of range of v2-only clients.
I think arguable that's OK/worth it, but it's "not [any] issues", no?
> However, this handful of corner cases should not block progress in the
> main cases.
What progress would be blocked?
I'm only talking about whether we choose to consider a "new graph" to be an:
<existing version number>
<existing chunk name (old content, possibly empty)>
<new chunk name (new content)>
v.s.:
<old/new version number>
<existing chunk name old/new (incompatible) content>
I.e. the "progress" this series is about is in getting the data locality
with smaller data with the new content.
But that's also possible to get with a very low amount of fixed-overhead.
Per the referenced E-Mail an "empty" commit-graph file was ~1k bytes in
2019, I haven't re-checked. In terms of wasted space it's miniscule &
<1/4 of one FS page on Linux.
I'm not just trying to rehash the same points, I *think* the version
bump is just an aesthetic choice & we're not getting any performance
difference out of that.
But I'm not sure from the "block progress" etc., so maybe I'm still
missing something...
>> I think that even if it's less conceptually clean it's worth considering
>> being over backwards to be kinder to such use-cases, unless it's really
>> required for other reasons to break such in-the-wild use-cases.
>>
>> Or in this case, if it's thought to be worth it to help reviewers decide
>> by separating the performance improvement aspect from the changed
>> interaction between new graphs and older clients.
>>
>> As a further nit on the proposed end-state here: Do I understand it
>> correctly that commitGraph.generationVersion=[1|2] (i.e. on current
>> "master") will always result in a file that's compatible with older
>> versions, since the only thing "v2" there controls now is to write the
>> optional GDAT and GDOV chunks?
>>
>> Whereas going from commitGraph.generationVersion=2 to
>> commitGraph.generationVersion=3 in this series will impact older clients
>> as noted above, since we're bumping the version (of the file, to 2 if
>> the config is 3, which as Junio noted is a bit confusing).
>>
>> I think if you're set on going down the path of bumping the top-level
>> version that deserves to be made much clearer in the added
>> documentation. Right now the only hint to that is a passing mention that
>> for v3:
>>
>> [it] will be incompatible with some old versions of Git
>>
>> Which if we're opting for breaking format changes really should note
>> some of the caveats above, that pre-v2.22.0 hard-dies, and probably
>> describe "some old versions of Git" a bit more clearly.
>>
>> It actually means once this gets released "the git version that was the
>> latest one you could download yesterday". Which a reader of the docs
>> probably won't expect when starting to play with this in mixed-version
>> environment.
>>
>> 1. https://lore.kernel.org/git/87h8acivkh.fsf@evledraar.gmail.com/
>
> This documentation could be altered to be specific about versions,
> but such a specific change makes assumptions of the version that will
> include it. As of now, the generation number v2 fixes will _probably_
> get in for 2.36 and the format change would have enough time to cook
> for 2.37, so I'll update the docs to refer to that version explicitly.
...
> The pre-2.22.0 change might be helpful to mention, but it could also be
> noise to the reader. We can revisit this when these patches are
> submitted again in another thread. There's also concern about third-
> party tools like libgit2. I'd rather draw the line as "tread carefully
> here" than "here is so much information that a reader might think it
> is all they need to know".
In terms of concern about libgit2 or any other implementation (which I
haven't looked at) isn't "tread carefully" to do it with new chunks if
possible, which we've done before with BIDX/BDAT, v.s. a version bump we
haven't done?
I'd think it wouldn't be an issue either way for any reader of the
format, and libgit2 is more specialized & won't have someone on RHEL6 or
whatever trying to inspect a random repo.
It just seems like a win-win to have a performance improvement with
smooth backwards compatibility v.s. without, if that's possible.
next prev parent reply other threads:[~2022-02-28 22:19 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-24 20:38 [PATCH 0/7] Commit-graph: Generation Number v2 Fixes, v3 implementation Derrick Stolee via GitGitGadget
2022-02-24 20:38 ` [PATCH 1/7] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-02-24 20:38 ` [PATCH 2/7] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-02-24 22:15 ` Junio C Hamano
2022-02-25 13:51 ` Derrick Stolee
2022-02-25 17:35 ` Junio C Hamano
2022-02-24 20:38 ` [PATCH 3/7] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-02-28 15:18 ` Patrick Steinhardt
2022-02-28 16:23 ` Derrick Stolee
2022-02-28 16:59 ` Patrick Steinhardt
2022-02-28 18:44 ` Derrick Stolee
2022-03-01 9:46 ` Patrick Steinhardt
2022-03-01 10:35 ` Patrick Steinhardt
2022-03-01 14:06 ` Derrick Stolee
2022-03-01 14:53 ` Patrick Steinhardt
2022-03-01 15:25 ` Derrick Stolee
2022-03-02 13:57 ` Patrick Steinhardt
2022-03-02 14:57 ` Derrick Stolee
2022-03-02 18:15 ` Junio C Hamano
2022-03-02 18:46 ` Derrick Stolee
2022-03-02 22:42 ` Junio C Hamano
2022-03-03 11:19 ` Patrick Steinhardt
2022-03-03 16:00 ` Derrick Stolee
2022-03-04 14:03 ` Derrick Stolee
2022-03-07 10:34 ` Patrick Steinhardt
2022-03-07 13:45 ` Derrick Stolee
2022-03-07 17:22 ` Junio C Hamano
2022-03-10 13:58 ` Patrick Steinhardt
2022-03-10 17:18 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 4/7] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
2022-02-24 22:35 ` Junio C Hamano
2022-02-25 13:53 ` Derrick Stolee
2022-02-25 17:38 ` Junio C Hamano
2022-02-24 20:38 ` [PATCH 5/7] commit-graph: document file format v2 Derrick Stolee via GitGitGadget
2022-02-24 22:55 ` Junio C Hamano
2022-02-25 22:31 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:44 ` Derrick Stolee
2022-02-28 14:27 ` Ævar Arnfjörð Bjarmason
2022-02-28 16:39 ` Derrick Stolee
2022-02-28 21:14 ` Ævar Arnfjörð Bjarmason [this message]
2022-03-01 14:19 ` Derrick Stolee
2022-03-01 14:29 ` Ævar Arnfjörð Bjarmason
2022-03-01 15:59 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 6/7] commit-graph: parse " Derrick Stolee via GitGitGadget
2022-02-24 23:01 ` Junio C Hamano
2022-02-25 13:54 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 7/7] commit-graph: write " Derrick Stolee via GitGitGadget
2022-02-24 21:42 ` [PATCH 0/7] Commit-graph: Generation Number v2 Fixes, v3 implementation Junio C Hamano
2022-02-24 23:06 ` Junio C Hamano
2022-02-25 13:55 ` Derrick Stolee
2022-02-28 13:53 ` [PATCH v2 0/4] Commit-graph: Generation Number v2 Fixes Derrick Stolee via GitGitGadget
2022-02-28 13:53 ` [PATCH v2 1/4] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-02-28 15:22 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:53 ` [PATCH v2 2/4] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-02-28 15:25 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:53 ` [PATCH v2 3/4] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-02-28 15:30 ` Ævar Arnfjörð Bjarmason
2022-02-28 16:43 ` Derrick Stolee
2022-02-28 13:53 ` [PATCH v2 4/4] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
2022-02-28 15:40 ` Ævar Arnfjörð Bjarmason
2022-03-01 17:23 ` [PATCH v2 0/4] Commit-graph: Generation Number v2 Fixes Ævar Arnfjörð Bjarmason
2022-03-01 19:48 ` [PATCH v3 0/5] " Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 1/5] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 2/5] t5318: extract helpers to lib-commit-graph.sh Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 3/5] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-03-01 20:13 ` Junio C Hamano
2022-03-01 20:30 ` Junio C Hamano
2022-03-02 14:13 ` Derrick Stolee
2022-03-01 19:48 ` [PATCH v3 4/5] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 5/5] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=220228.86ilsy3a8b.gmgdl@evledraar.gmail.com \
--to=avarab@gmail.com \
--cc=abhishekkumar8222@gmail.com \
--cc=derrickstolee@github.com \
--cc=git@vger.kernel.org \
--cc=gitgitgadget@gmail.com \
--cc=gitster@pobox.com \
--cc=me@ttaylorr.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).