Horton

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

Jeff Davis <pgsql@j-davis.com>

View thread
From: Jeff Davis <pgsql@j-davis.com>
To: Andres Freund <andres@anarazel.de>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Dilip Kumar <dilipbalaut@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, pgsql-hackers@lists.postgresql.org, Nathan Bossart <nathandbossart@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: 2024-02-12T20:46:00Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add XLogCtl->logInsertResult

  2. Add assert to WALReadFromBuffers().

  3. Read WAL directly from WAL buffers.

  4. Additional write barrier in AdvanceXLInsertBuffer().

  5. Use 64-bit atomics for xlblocks array elements.

  6. Don't trust unvalidated xl_tot_len.

On Mon, 2024-02-12 at 12:18 -0800, Andres Freund wrote:
> +    upto = Min(startptr + count, LogwrtResult.Write);
> +    nbytes = upto - startptr;
> 
> Shouldn't it pretty much be a bug to ever encounter this?

In the current code it's impossible, though Bharath hinted at an
extension which could reach that path.

What I committed was a bit of a compromise -- earlier versions of the
patch supported reading right up to the Insert pointer (which requires
a call to WaitXLogInsertionsToFinish()). I wasn't ready to commit that
code without seeing a more about how that would be used, but I thought
it was reasonable to have some simple code in there to allow reading up
to the Write pointer.

It seems closer to the structure that we will ultimately need to
replicate unflushed data, right?

Regards,
	Jeff Davis

[1]
https://www.postgresql.org/message-id/CALj2ACW65mqn6Ukv57SqDTMzAJgd1N_AdQtDgy+gMDqu6v618Q@mail.gmail.com