Re: Improve WALRead() to suck data directly from WAL buffers when possible

On Wed, Feb 14, 2024 at 6:59 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>
> Attached 2 patches.
>
> Per Andres's suggestion, 0001 adds an:
> Assert(startptr + count <= LogwrtResult.Write)
>
> Though if we want to allow the caller (e.g. in an extension) to
> determine the valid range, perhaps using WaitXLogInsertionsToFinish(),
> then the check is wrong.

Right.

> Maybe we should just get rid of that code
> entirely and trust the caller to request a reasonable range?

I'd suggest we strike a balance here - error out in assert builds if
startptr+count is past the current insert position and trust the
callers for production builds. It has a couple of advantages over
doing just Assert(startptr + count <= LogwrtResult.Write):
1) It allows the caller to read unflushed WAL directly from WAL
buffers, see the attached 0005 for an example.
2) All the existing callers where WALReadFromBuffers() is thought to
be used are ensuring WAL availability by reading upto the flush
position so no problem with it.

Also, a note before WALRead() stating the caller must request the WAL
at least that's written out (upto LogwrtResult.Write). I'm not so sure
about this, perhaps, we don't need this comment at all.

Here, I'm with v23 patch set:

0001 - Adds assertion in WALReadFromBuffers() to ensure the requested
WAL isn't beyond the current insert position.
0002 - Adds a new test module to demonstrate how one can use
WALReadFromBuffers() ensuring WaitXLogInsertionsToFinish() if need be.
0003 - Uses WALReadFromBuffers in more places like logical walsenders
and backends.
0004 - Removes zero-padding related stuff as discussed in
https://www.postgresql.org/message-id/CALj2ACWBRFac2TingD3PE3w2EBHXUHY3=AEEZPJmqhpEOBGExg@mail.gmail.com.
This is needed in this patch set otherwise the assertion added in 0001
fails after 0003.
0005 - Adds a page_read callback for reading from WAL buffers in the
new test module added in 0002. Also, adds tests.

Thoughts?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com