Subject: Re: [PATCH 5/7] commit-graph: document file format v2

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 28 Feb 2022 22:14:16 +0100

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/[email-masked]/
>
> 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.