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[24.132.120.189]) by smtp.gmail.com with ESMTPSA id u5-20020a170906780500b006d0b99162casm4465680ejm.114.2022.02.28.07.19.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Feb 2022 07:19:18 -0800 (PST) Received: from avar by gmgdl with local (Exim 4.95) (envelope-from ) id 1nOhnx-001QLH-Oq; Mon, 28 Feb 2022 16:19:17 +0100 From: =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason To: Derrick Stolee Cc: Derrick Stolee via GitGitGadget , 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 15:27:49 +0100 References: <7f9b65bd22551fd7fd5d2f0bf18aee8c25f1db99.1645735117.git.gitgitgadget@gmail.com> <220225.86a6ee7eid.gmgdl@evledraar.gmail.com> User-agent: Debian GNU/Linux bookworm/sid; Emacs 27.1; mu4e 1.6.10 In-reply-to: Message-ID: <220228.86pmn73toq.gmgdl@evledraar.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Mon, Feb 28 2022, Derrick Stolee wrote: > On 2/25/2022 5:31 PM, =C3=86var Arnfj=C3=B6r=C3=B0 Bjarmason wrote: >>=20 >> On Thu, Feb 24 2022, Derrick Stolee via GitGitGadget wrote: >>=20 > ... >>> Generation Data (ID: {'G', 'D', 'A', 'T' }) (N * 4 bytes) [Optional] >>> * This list of 4-byte values store corrected commit date offsets f= or the >>> @@ -103,6 +112,9 @@ CHUNK DATA: >>> * Generation Data chunk is present only when commit-graph file is = written >>> by compatible versions of Git and in case of split commit-graph = chains, >>> the topmost layer also has Generation Data chunk. >>> + * This chunk does not exist if the commit-graph file format versio= n is 2, >>> + because the corrected commit date offset data is stored in the C= ommit >>> + Data chunk. >>>=20=20 >>> Generation Data Overflow (ID: {'G', 'D', 'O', 'V' }) [Optional] >>> * This list of 8-byte values stores the corrected commit date offs= ets >>=20 >> We talked a while ago now about how we do commit-graph format changes >> and this is partially echoing those earlier questions[1] from 2019. >>=20 >> I fully understand why we're writing this amended CDAT chunk in a >> different layout. By not having the GDAT side-chunk to look up in the >> data is more local, that part of the file is more compact etc. >>=20 >> What I don't understand is why getting those performance improvements >> requires the breaking version change & the writing of the incompatible >> version number. >>=20 >> I.e. couldn't the differently formatted CDAT chunk be written instead to= a new >> chunk name (say "2DAT") instead? Per [1] we'd pay a small fixed cost for >> a possibly empty chunk (I didn't re-do those numbers), but surely the >> performance improvements will be about the same for that miniscule >> overhead. > > CDAT is a required chunk. It is part of the v1 spec that CDAT exists > and is correct. All other Git clients will error out when reading a > "v1" graph without such a chunk, and in a way that is less helpful to > users. Instead of clearly indicating "file version is too new" it will > say "commit-graph is missing the Commit Data chunk" which is not > helpful. Yes. That would be the worst of both worlds. I thought the reference to the 2019-era post made it clear (which is explicit about this aspect), but I'm talking about writing one of: A. An empty chunk B. Keeping a "stale" chunk around (as we re-write the graph) C. Duplicate writes of new/old chunks. And not simply omitting the CDAT chunk. As you point out would give you all the drawbacks of a version number change, with none of the benefits. I haven't re-tested this now, but at the time doing any of (A..C) would work smoothly for older clients, while giving newer ones improved data. >> It will give you something you can't have here, which is optional >> compatibility with older clients by writing both versions. That'll be a >> ~2x as large file on disk, but with the page cache & each client version >> skipping to the data it needs caching characteristics & data locality >> should work out to about the same thing. > > Writing both is the only way that this could work without incrementing > the graph version number, but I'd rather just update the number and > avoid wasting the effort to write that extra data. ... > It seems you are hyper-focused on "we don't _need_ to update the version > number" and you are willing to recommend wasteful approaches in order to > support that stance. I'd say less hyper-focused, and more clarifying an IMO major unstated trade-off of the proposed format change. > So: you're right. We don't _need_ to update the version number. But this > is the best choice among the options available. ... >> 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". 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=20 operator started writing a new version, but forgot about a set of machines that are still on the old version. 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=3D[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=3D2 to commitGraph.generationVersion=3D3 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/