Re: [PATCH v3 0/4] A design for future-proofing fsync() configuration

[Neeraj Singh <nksingh85@gmail.com>]

Hello Everyone,
I wanted to revive this thread in the new year.

To summarize the current state of affairs:
* The current fsync patch series implements two new configuration options:
       core.fsync = <comma-separate list> -- select which repo
components will be fsynced
       core.fsyncMethod = fsync|writeout-only  -- select what form of
fsyncing will be done

* This patch series now ignores core.fsyncObjectFiles with a
deprecation warning pointing the user at core.fsync.

* There is a follow-on series that will extend the core.fsyncMethod to
also include a `batch` mode that speeds up bulk operations by avoiding
repeated disk cache flushes.

* I developed the current mechanism after Ævar pointed out that the
original `core.fsyncObjectFiles=batch` change would cause older
versions of Git to die() when exposed to a new configuration. There
were also several fsync changes floating around, including Patrick
Steinhardts `core.fsyncRefFiles` change [1] and Eric Wong's
`core.fsync = false` change [2].

* The biggest sticking points are in [3].  The fundamental
disagreement is about whether core.fsync should look like:
      A) core.fsync = objects,commit-graph   [current patch implementation]
      or
      B) core.fsync = objects
          core.fsync = commit-graph    [Ævar's multivalued proposal].
I prefer sticking with (A) for reasons spelled out in the thread. I'm
happy to re-litigate this discussion though.

* There's also a sticking point about whether we should fsync when
invoking pack-objects against stdout.  I think that mostly reflects a
missing comment in the code rather than a real disagreement.

* Now that ew/test-wo-fsync has been integrated, there's some
redundancy between core.fsync=none and Eric's patch.

Open questions:
1) What format should we use for the core.fsync configuration to
select individual repo components to sync?
2) Are we okay with deprecating core.fsyncObjectFiles in a single
release with a warning?
3) Is it reasonable to expect people adding new persistent files to
add and document new values of the core.fsync settings?

Thanks,
Neeraj

[1]  https://lore.kernel.org/git/20211030103950.M489266@dcvr/
[2] https://lore.kernel.org/git/20211028002102.19384-1-e@80x24.org/
[3] https://lore.kernel.org/git/211207.86wnkgo9fv.gmgdl@evledraar.gmail.com/