Horton

Re: Berserk Autovacuum (let's save next Mandrill)

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: David Rowley <dgrowleyml@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Darafei Komяpa Praliaskouski <me@komzpa.net>, Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org, Alvaro Herrera <alvherre@2ndquadrant.com>, Michael Banck <mbanck@gmx.net>
Date: 2020-03-16T04:34:25Z
Lists: pgsql-hackers 
On Fri, Mar 13, 2020 at 10:48:27PM +0100, Laurenz Albe wrote:
> On Fri, 2020-03-13 at 13:44 -0500, Justin Pryzby wrote:
> > Possible it would be better to run VACUUM *without* freeze_min_age=0 ?  (I get
> > confused and have to spend 20min re-reading the vacuum GUC docs every time I
> > deal with this stuff, so maybe I'm off).
> > 
> > As I understand, the initial motivation of this patch was to avoid disruptive
> > anti-wraparound vacuums on insert-only table.  But if vacuum were triggered at
> > all, it would freeze the oldest tuples, which is all that's needed; especially
> > since fd31cd2651 "Don't vacuum all-frozen pages.", those pages would never need
> > to be vacuumed again.  Recently written tuples wouldn't be frozen, which is ok,
> > they're handled next time.
> 
> Freezing tuples too early is wasteful if the tuples get updated or deleted
> soon after, but based on the assumption that an autovacuum triggered by insert
> is dealing with an insert-mostly table, it is not that wasteful.

You're right that it's not *that* wasteful.  If it's a table that gets 90%
inserts/10% updates, then only 10% of its tuples will be frozen.  In the worst
case, it's the same tuples every time, and that's somewhat wasteful.  In the
best case, those tuples are clustered on a small number of pages.

-- 
Justin

Commits

  1. Further improve stability fix for partition_aggregate test.

  2. Improve stability fix for partition_aggregate test.

  3. Attempt to stabilize partitionwise_aggregate test

  4. Fix race condition in statext_store().

  5. Attempt to fix unstable regression tests, take 2

  6. Attempt to fix unstable regression tests

  7. Trigger autovacuum based on number of INSERTs

  8. Fix upper limit for vacuum_cleanup_index_scale_factor