To: Greg Smith <gsmith@gregsmith.com>
cc: Gauri Kanekar <meetgaurikanekar@gmail.com>,
	pgsql-performance@postgresql.org
Subject: Re: Replication Syatem 
In-reply-to: <Pine.GSO.4.64.0804290347560.20575@westnet.com> 
References: <7e4ba9550804280638ie81dc23w472785c83cd38fcf@mail.gmail.com>
	<a2de01dd0804280658t68ac22dsf182d304a94de5a2@mail.gmail.com>
	<7e4ba9550804280705q43c51e66l104a5ff9272bbd4d@mail.gmail.com>
	<20080428162243.GN2970@commandprompt.com>
	<7e4ba9550804282246p73c62fc6y5e7cb7dda55a6b50@mail.gmail.com>
	<Pine.GSO.4.64.0804290347560.20575@westnet.com>
Comments: In-reply-to Greg Smith <gsmith@gregsmith.com>
	message dated "Tue, 29 Apr 2008 04:37:07 -0400"
Date: Tue, 29 Apr 2008 10:16:02 -0400
Message-ID: <873.1209478562@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Greg Smith <gsmith@gregsmith.com> writes:
> The model here assumes that you'll need that space again for the next time 
> you UPDATE or INSERT a row.  So instead VACUUM just keeps those available 
> for database reuse rather than returning it to the operating system.

> Now, if you don't VACUUM frequently enough, this model breaks down, and 
> the table can get bigger with space that may never get reused.  The idea 
> is that you should be VACUUMing up now unneeded rows at about the same 
> rate they're being re-used.  When you don't keep up, the database can 
> expand in space that you don't get back again.  The right answer to this 
> problem is not to use VACUUM FULL; it's to use regular VACUUM more often.

Also, you need to make sure you have the FSM parameters set high enough
so that all the free space found by a VACUUM run can be remembered.

The less often you run VACUUM, the more FSM space you need, because
there'll be more free space reclaimed per run.

			regards, tom lane