Horton

Re: Use WALReadFromBuffers in more places

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Jingtang Zhang <mrdrivingduck@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org, Nitin Jadhav <nitinjadhavpostgres@gmail.com>
Date: 2025-09-22T15:26:28Z
Lists: pgsql-hackers

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  1. Read WAL directly from WAL buffers.

On Sat, 2025-09-13 at 22:04 -0700, Bharath Rupireddy wrote:
> 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.

IIRC, one motivation (perhaps the primary motivation?) was to make it
possible to read buffers before they are flushed. It was always
possible to read already-flushed buffers.

The benefit of reading unflushed buffers is that we can replicate the
WAL sooner (though it can't be replayed until the primary flushes it).
Is that right?

Regards,
	Jeff Davis