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Subject: Re: Use WALReadFromBuffers in more places
From: Jingtang Zhang <mrdrivingduck@gmail.com>
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Date: Sat, 20 Sep 2025 16:33:20 +0800
Cc: pgsql-hackers@lists.postgresql.org,
 Nitin Jadhav <nitinjadhavpostgres@gmail.com>
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To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
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Hi~

> Thanks for looking at this. Yes, the WAL writers can zero out flushed
> buffers before WALReadFromBuffers gets to them. However,
> WALReadFromBuffers was intentionally designed as an opportunistic
> optimization - it's a "try this first, quickly" approach before
> falling back to reading from WAL files. The no-locks design ensures it
> never gets in the way of backends generating WAL, which is critical
> for overall system performance.

Yes, it is actually an interesting thing, beyond current topic. Since
we are using buffered I/O, even though we cannot read from WAL buffer =
due
to the opportunistic AdvanceXLInsertBuffer by WAL writer, later WALRead =
may
still find the page inside OS page cache, with high probability, because =
the
page has just been written out. So WALRead will be fast, too.

But if we are moving forward to direct I/O some day in the future, the =
cost
of WALReadFromBuffers and WALRead might be obvious. Maybe the =
opportunistic
WAL buffer initialization could keep a small ratio of old pages inside =
WAL
buffer so these pages can still be hit by WALReadFromBuffers.

> I rebased and attached the v3 patch.

The v3 patch LGTM.

=E2=80=94
Regards,
Jingtang

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