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[24.132.120.189]) by smtp.gmail.com with ESMTPSA id me14-20020a170906aece00b006d6e43b3cb1sm134630ejb.44.2022.02.28.14.19.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Feb 2022 14:19:32 -0800 (PST) Received: from avar by gmgdl with local (Exim 4.95) (envelope-from ) id 1nOoMe-001cE1-1x; Mon, 28 Feb 2022 23:19:32 +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 22:14:16 +0100 References: <7f9b65bd22551fd7fd5d2f0bf18aee8c25f1db99.1645735117.git.gitgitgadget@gmail.com> <220225.86a6ee7eid.gmgdl@evledraar.gmail.com> <220228.86pmn73toq.gmgdl@evledraar.gmail.com> User-agent: Debian GNU/Linux bookworm/sid; Emacs 27.1; mu4e 1.6.10 In-reply-to: Message-ID: <220228.86ilsy3a8b.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 2/28/2022 9:27 AM, =C3=86var Arnfj=C3=B6r=C3=B0 Bjarmason wrote: >>=20 >> On Mon, Feb 28 2022, Derrick Stolee wrote: >>=20 >>> On 2/25/2022 5:31 PM, =C3=86var Arnfj=C3=B6r=C3=B0 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. >>=20 >> 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]. >>=20 >> 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? >>=20 >> 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 :) >>=20 >> 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. >>=20 >> But git is used by *a lot* of people in a bunch of different >> scenarios. E.g.: >>=20 >> * A shared (hopefully read-only) NFS mounted by remote "unmanaged" clie= nts. >> * A tarred-up directory including a .git, which may be transferred to >> a machine with a pre-v2.22.0 version. >>=20 >> Or even softer cases of failure, such as: >>=20 >> * 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. > > 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: v.s.: 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. >>=20 >> 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. >>=20 >> 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? >>=20 >> Whereas going from commitGraph.generationVersion=3D2 to >> commitGraph.generationVersion=3D3 in this series will impact older clien= ts >> 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). >>=20 >> 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: >>=20 >> [it] will be incompatible with some old versions of Git >>=20 >> 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. >>=20 >> 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. >>=20 >> 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.