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Re: autovacuum not prioritising for-wraparound tables

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Kevin Grittner <kgrittn(at)ymail(dot)com>
Cc: Andres Freund <andres(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: autovacuum not prioritising for-wraparound tables
Date: 2013-01-30 15:58:07
Message-ID: CAMkU=1zdxunXAigyrwqsQ9f75Pgtpz3YEnhoWQYyaeggnZ1PHA@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Jan 30, 2013 at 5:39 AM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>>
>> Don't think I did. I was talking about vacuum_freeze_table_age
>> because that influences the amount of full-table scans
>
> Not any more than vacuum_freeze_min_age does.

There is a lot more room for vacuum_freeze_table_age to be increased,
then there is for vacuum_freeze_min_age to be decreased.

> http://www.postgresql.org/docs/9.2/interactive/routine-vacuuming.html#AUTOVACUUM
>
> | a whole table sweep is forced if the table hasn't been fully
> | scanned for vacuum_freeze_table_age minus vacuum_freeze_min_age
> | transactions.

Why is vacuuming described as such a violent process?  It doesn't
"force" a table sweep, it just goes ahead and performs one.  In
general, it seems hard to tell from the docs that this only promotes a
vacuum which is going to happen anyway from a vm one to a full scan
one.  The forcefulness makes it sound more like it is doing vacuums
that wouldn't happen otherwise (like autovacuum_freeze_max_age does,
which actually could be described as "force" since it turns on the
autovac launcher even if it is configured to be off)

> So reducing vacuum_freeze_min_age not only helps minimize the
> writes that are needed when autovacuum needs to scan the entire
> heap,

How does it do that? If the tuple doesn't need to frozen now because
it was already frozen, that just means the write happened at a
different time but it still happened.

Cheers,

Jeff


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