Horton

Re: Option to not use ringbuffer in VACUUM, using it in failsafe mode

David G. Johnston <david.g.johnston@gmail.com>

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From: "David G. Johnston" <david.g.johnston@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Peter Geoghegan <pg@bowt.ie>, pgsql-hackers@postgresql.org, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, David Rowley <dgrowleyml@gmail.com>
Date: 2023-01-12T00:26:19Z
Lists: pgsql-hackers 
On Wed, Jan 11, 2023 at 2:39 PM Andres Freund <andres@anarazel.de> wrote:

> Hi,
>
> On 2023-01-11 16:18:34 -0500, Tom Lane wrote:
> > Peter Geoghegan <pg@bowt.ie> writes:
> > > On Wed, Jan 11, 2023 at 11:18 AM Andres Freund <andres@anarazel.de>
> wrote:
> > >> I don't like that - it's also quite useful to disable use of
> ringbuffers when
> > >> you actually need to clean up indexes. Especially when we have a lot
> of dead
> > >> tuples we'll rescan indexes over and over...
> >
> > > That's a fair point.
> >
> > > My vote goes to "REUSE_BUFFERS", then.
> >
> > I wonder whether it could make sense to allow a larger ringbuffer size,
> > rather than just the limit cases of "on" and "off".
>
> I can see that making sense, particularly if we were to later extend this
> to
> other users of ringbuffers. E.g. COPYs us of the ringbuffer makes loading
> of
> data > 16MB but also << s_b vastly slower, but it can still be very
> important
> to use if there's lots of parallel processes loading data.
>
>
Should we just add "ring_buffers" to the existing "shared_buffers" and
"temp_buffers" settings?

Then give VACUUM a (BUFFER_POOL=ring*|shared) option?

I think making DBAs aware of this dynamic and making the ring buffer usage
user-facing is beneficial in its own right (at least, the concept that
changes done by vacuum don't impact shared_buffers, regardless of how that
non-impact manifests).  But I don't see much benefit trying to come up with
a different name.

David J.

Commits

  1. Add unit to vacuum_buffer_usage_limit value in postgresql.conf.sample.

  2. Improve VACUUM/ANALYZE BUFFER_USAGE_LIMIT docs

  3. Add --buffer-usage-limit option to vacuumdb

  4. Add VACUUM/ANALYZE BUFFER_USAGE_LIMIT option

  5. Move various prechecks from vacuum() into ExecVacuum()

  6. Always make a BufferAccessStrategy for ANALYZE

  7. Rename BufferAccessStrategyData.ring_size to nbuffers

  8. Disable vacuum's use of a buffer access strategy during failsafe

  9. Only make buffer strategy for vacuum when it's likely needed

  10. Remove some global variables from vacuum.c

  11. Doc: add Buffer Access Strategy to the glossary