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

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 28 Feb 2022 15:27:49 +0100

On Mon, Feb 28 2022, Derrick Stolee wrote:

> On 2/25/2022 5:31 PM, Ævar Arnfjörð Bjarmason wrote:
>> 
>> On Thu, Feb 24 2022, Derrick Stolee via GitGitGadget wrote:
>> 
> ...
>>>    Generation Data (ID: {'G', 'D', 'A', 'T' }) (N * 4 bytes) [Optional]
>>>      * This list of 4-byte values store corrected commit date offsets for 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 version is 2,
>>> +      because the corrected commit date offset data is stored in the Commit
>>> +      Data chunk.
>>>  
>>>    Generation Data Overflow (ID: {'G', 'D', 'O', 'V' }) [Optional]
>>>      * This list of 8-byte values stores the corrected commit date offsets
>> 
>> 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.
>> 
>> 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.
>> 
>> What I don't understand is why getting those performance improvements
>> requires the breaking version change & the writing of the incompatible
>> version number.
>> 
>> 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 
   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=[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]/