Date: Wed, 3 Feb 2021 20:07:16 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Multiple full page writes in a single checkpoint?
Message-ID: <20210204010716.GD11069@momjian.us>
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On Wed, Feb  3, 2021 at 05:00:19PM -0800, Andres Freund wrote:
> Hi,
> 
> On 2021-02-03 19:21:25 -0500, Bruce Momjian wrote:
> > On Wed, Feb  3, 2021 at 03:29:13PM -0800, Andres Freund wrote:
> > > Changing this is *completely* infeasible. In a lot of workloads it'd
> > > cause a *massive* explosion of WAL volume. Like quadratically. You'll
> > > need to find another way to generate a nonce.
> >
> > Do we often do multiple writes to the file system of the same page
> > during a single checkpoint, particularly only-hint-bit-modified pages?
> > I didn't think so.
> 
> It can easily happen. Consider ringbuffer using scans (like vacuum,
> seqscan) - they'll force the buffer out to disk soon after it's been
> dirtied. And often will read the same page again a short bit later. Or
> just any workload that's a bit bigger than shared buffers (but data is
> in the OS cache).  Subsequent scans will often have new hint bits to
> set.

Oh, good point.

> > Is the logical approach here to modify XLogSaveBufferForHint() so if a
> > page write is not needed, to create a dummy WAL record that just
> > increments the WAL location and updates the page LSN?
> > (Is there a small WAL record I should reuse?)
> 
> I think an explicit record type would be better. Or a hint record
> without an associated FPW.

OK.

> > I can try to add a hint-bit-page-write page counter, but that might
> > overflow, and then we will need a way to change the LSN anyway.
> 
> That's just a question of width...

Yeah, the hint bit counter is just delaying the inevitasble, plus it
changes the page format, which I am trying to avoid.  Also, I need this
dummy record only if the page is marked clean, meaning a write
to the file system already happened in the current checkpoint --- should
not be to bad.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee