Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: "David G. Johnston" <david.g.johnston@gmail.com>
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: 2023-01-12T00:36:38Z
Lists: pgsql-hackers 
Hi,

On 2023-01-11 17:26:19 -0700, David G. Johnston wrote:
> Should we just add "ring_buffers" to the existing "shared_buffers" and
> "temp_buffers" settings?

The different types of ring buffers have different sizes, for good reasons. So
I don't see that working well. I also think it'd be more often useful to
control this on a statement basis - if you have a parallel import tool that
starts NCPU COPYs you'd want a smaller buffer than a single threaded COPY. Of
course each session can change the ring buffer settings, but still.


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

That seems likely to mislead, because it'd still use shared buffers when the
blocks are already present. The ring buffers aren't a separate buffer pool,
they're a subset of the normal bufferpool. Lookup is done normally, only when
a page isn't found, the search for a victim buffer first tries to use a buffer
from the ring.


> 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).

VACUUM can end up dirtying all of shared buffers, even with the ring buffer in
use...

Greetings,

Andres Freund

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