Horton

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

Melanie Plageman <melanieplageman@gmail.com>

View thread View on postgr.es
From: Melanie Plageman <melanieplageman@gmail.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Peter Geoghegan <pg@bowt.ie>, pgsql-hackers@postgresql.org, Robert Haas <robertmhaas@gmail.com>
Date: 2023-04-01T17:29:13Z
Lists: pgsql-hackers 
Hi,

I was just doing some cleanup on the main patch in this set and realized
that it was missing a few things. One of which is forbidding the
BUFFER_USAGE_LIMIT with VACUUM FULL since VACUUM FULL does not use a
BAS_VACUUM strategy.

VACUUM FULL technically uses a bulkread buffer access strategy for
reading the original relation if its number of blocks is > number of
shared buffers / 4 (see initscan()). The new rel writing is done using
smgrextend/write directly and doesn't go through shared buffers. I
think it is a stretch to try and use the size passed in to VACUUM by
BUFFER_USAGE_LIMIT for the bulkread strategy ring.

As for forbidding the combination, I noticed that when VACUUM FULL is
specified with INDEX_CLEANUP OFF, there is no syntax error but the
INDEX_CLEANUP option is simply ignored. This is documented behavior.

I somehow feel like VACUUM (FULL, BUFFER_USAGE_LIMIT 'x') should error
out instead of silently not using the buffer usage limit, though.

I am looking for others' opinions.

- Melanie

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