Message-ID: <4677E2E1.2060902@metacarta.com>
Date: Tue, 19 Jun 2007 10:06:25 -0400
From: Karl Wright <kwright@metacarta.com>
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To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-performance@postgresql.org
Subject: Re: Performance query about large tables, lots of concurrent
 access
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	<20070619132643.GC4265@alvh.no-ip.org>
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Alvaro Herrera wrote:
> Karl Wright wrote:
>> Alvaro Herrera wrote:
>>> Karl Wright wrote:
>>>
>>>> This particular run lasted four days before a VACUUM became essential. 
>>>> The symptom that indicates that VACUUM is needed seems to be that the 
>>>> CPU usage of any given postgresql query skyrockets.  Is this essentially 
>>>> correct?
>>> Are you saying you weren't used to run VACUUM all the time?  If so,
>>> that's where the problem lies.
>> Postgresql 7.4 VACUUM runs for so long that starting it with a cron job 
>> even every 24 hours caused multiple instances of VACUUM to eventually be 
>> running in my case.  So I tried to find a VACUUM schedule that permitted 
>> each individual vacuum to finish before the next one started.  A vacuum 
>> seemed to require 4-5 days with this particular database - or at least 
>> it did for 7.4.  So I had the VACUUM schedule set to run every six days.
> 
> How large is the database?  I must admit I have never seen a database
> that took 4 days to vacuum.  This could mean that your database is
> humongous, or that the vacuum strategy is wrong for some reason.
> 

The database is humongus, and the machine is under intense load.  On the 
instance where this long vacuum occurred, there were several large 
tables - one with 7,000,000 rows, one with 14,000,000, one with 
140,000,000, and one with 250,000,000.

> You know that you can run vacuum on particular tables, right?  It would
> be probably a good idea to run vacuum on the most updated tables, and
> leave alone those that are not or little updated (hopefully the biggest;
> this would mean that an almost-complete vacuum run would take much less
> than a whole day).

Yeah, sorry, that doesn't apply here.

> 
> Or maybe vacuum was stuck waiting on a lock somewhere.
> 
>> I will be experimenting with 8.1 to see how long it takes to complete a 
>> vacuum under load conditions tonight.
> 
> You can also turn autovacuum on in 8.1, which might help quite a bit
> with finding a good vacuum schedule (you would need a bit of tuning it
> though, of course).
> 
> In any case, if you are struggling for performance you are strongly
> adviced to upgrade to 8.2.
> 

Ok - that's something I should be able to do once we can go to debian's 
etch release.  There's a backport of 8.2 available there.  (The one for 
sarge is still considered 'experimental').

Karl