Subject: Re: [PATCH 00/30] [RFC] extensions.refFormat and packed-refs v2 file format

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 02 Dec 2022 19:24:34 +0100

On Fri, Dec 02 2022, Han-Wen Nienhuys wrote:

> On Thu, Dec 1, 2022 at 9:19 PM Derrick Stolee <[email-masked]> wrote:
>> >> The reason to start with this step is that the benefits and risks are
>> >> clearly understood, which can motivate us to establish the mechanism for
>> >> changing the ref format by defining the extension.
>> >
>> > I believe that the v2 format is a safe change with performance
>> > improvements, but it's a backward incompatible format change with only
>> > modest payoff. I also don't understand how it will help you do a stack
>> > of tables,
>> > which you need for your primary goal (ie. transactions/deletions
>> > writing only the delta, rather than rewriting the whole file?).
>>
>> The v2 format doesn't help me on its own, but it has other benefits
>> in terms of size and speed, as well as the "ref count" functionality.
>>
>> The important thing is that the definition of extensions.refFormat
>> that I'm proposing in this RFC establishes a way to make incremental
>> progress on the ref format, allowing the stacked format to come in
>> later with less friction.
>
> I guess you want to move the read/write stack under the loose storage
> (packed backend), and introduce (read loose/packed + write packed
> only) mode that is transitional?
>
> Before you embark on this incremental route, I think it would be best
> to think through carefully how an online upgrade would work in detail
> (I think it's currently not specified?) If ultimately it's not
> feasible to do incrementally, then the added complexity of the
> incremental approach will be for naught.
>
> The incremental mode would only be of interest to hosting providers.
> It will only be used transitionally. It is inherently going to be
> complex, because it has to consider both storage modes at the same
> time, and because it is transitional, it will get less real life
> testing. At the same time, the ref database is comparatively small, so
> the availability blip that converting the storage offline will impair
> is going to be small. So, the incremental approach is rather expensive
> for a comparatively small benefit.
>
> I also thought a bit about how you could make the transition seamless,
> but I can't see a good way: you have to coordinate between tables.list
> (the list of reftables active or whatever file signals the presence of
> a stack) and files under refs/heads/. I don't know how to do
> transactions across multiple files without cooperative locking.

A multi-backend transaction would be hard to do at the best of times,
but we'd also presumably run into the issue that not all ref operations
currently use the transaction mechanism (e.g. branch copying/moving). So
if one or the other fails there all bets are off as far as getting back
to a consistent state.

Perhaps a more doable & interesting approach would be to have a "slave"
backend that would follow along, i.e. we'd replay all operations from
"master" to "slave" (as with DB replication, just within a single
repository).

We might get out of sync, but as the "master" is always the source of
truth presumably we could run some one-off re-exporting of the refspace
get back up-to-date, and hopefully not get out of sync again.

Then once we're ready, we could flip the switch indicating what becomes
the canonical backend.

For reftable the FS layout under .git/* is incompatible, so we'd also
need to support writing to some alternate directory to make such a thing
work...