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Re: controlling autovacuum during the day.

From: Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Michael Fuhr <mike(at)fuhr(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, John Lister <john(dot)lister-ps(at)kickstone(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: Re: controlling autovacuum during the day.
Date: 2008-12-17 19:35:51
Message-ID: 1229542551.6302.13.camel@bnicholson-desktop (view raw or flat)
Thread:
Lists: pgsql-admin
On Wed, 2008-12-17 at 13:31 -0300, Alvaro Herrera wrote:
> Michael Fuhr wrote:
> > On Wed, Dec 17, 2008 at 09:47:23AM -0500, Tom Lane wrote:
> > > "John Lister" <john(dot)lister-ps(at)kickstone(dot)com> writes:
> > > > Cheers for the quick reply. I've tweaked them quite a bit, but we have quite 
> > > > a few heavily updated tables that i'd like vacuuming to keep them in check. 
> > > > Unfortunately the autovacuum does a FULL vacuum every so often locking the 
> > > > tables for quite a long time, i'd like to move these to the evening if 
> > > > possible.
> > > 
> > > Huh?  Autovacuum *never* does VACUUM FULL.
> > 
> > Perhaps autovacuum is shrinking the table after finding lots of empty
> > pages at the end, as in what VACUUM VERBOSE is logging here:
> > 
> > INFO:  "foo": truncated 11944384 to 8877366 pages
> > 
> > I think this acquires an AccessExclusiveLock.  I've seen this take
> > hours in the case of a table with a lot of empty pages.
> 
> There was a bug in earlier versions which caused lazy vacuum to do
> cost-based delays during this phase, which caused the lock to be held
> for ridiculous lengths of time.  This was fixed in 8.2.something; it
> shouldn't sleep anymore, but it does need to scan those pages in order
> to truncate, so it can still take a while.

Does this mean that if you are using autovac on 8.1, you should not use
the cost delay feature?
-- 
´╗┐Brad Nicholson  416-673-4106
Database Administrator, Afilias Canada Corp.


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