Message-ID: <4AF9A425.9090102@2ndquadrant.com>
Date: Tue, 10 Nov 2009 12:34:29 -0500
From: Greg Smith <greg@2ndquadrant.com>
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To: Laurent Laborde <kerdezixe@gmail.com>
CC: Ivan Voras <ivoras@freebsd.org>, pgsql-performance@postgresql.org
Subject: Re: limiting performance impact of wal archiving.
References: <8a1bfe660911100355l34a557ffj8244079c1fb689b5@mail.gmail.com>	
	<hdbrv8$p35$1@ger.gmane.org>	
	<8a1bfe660911100700l2a44f638m97ea6136b65deb21@mail.gmail.com>	
	<hdbvqt$7v8$1@ger.gmane.org>	
	<8a1bfe660911100729q42ac732cga9bbadd52e2533dd@mail.gmail.com>	
	<4AF99666.5090705@2ndquadrant.com>
	<8a1bfe660911100852l5573ab30ne91c6d8937dc836b@mail.gmail.com>
In-Reply-To: <8a1bfe660911100852l5573ab30ne91c6d8937dc836b@mail.gmail.com>
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Laurent Laborde wrote:
> On Tue, Nov 10, 2009 at 5:35 PM, Greg Smith <greg@2ndquadrant.com> wrote:
>   
> I have 1 spare dedicated to hot standby, doing nothing but waiting for
> the master to fail.
> + 2 spare candidate for cluster mastering.
>
> In theory, i could even disable fsync and all "safety" feature on the master.
>   
There are two types of safety issues here:

1) Will the database be corrupted if there's a crash?  This can happen 
if you turn off fsync, and you'll need to switch to a standby to easily 
get back up again

2) Will you lose transactions that have been reported as committed to a 
client if there's a crash?  This you're exposed to if synchronous_commit 
is off, and whether you have a standby or not doesn't change that fact.

> Everybody told me "nooo ! LVM is ok, no perceptible overhead, etc ...)
> Are you 100% about LVM ? I'll happily trash it :)
Believing what people told you is how you got into trouble in the first 
place.  You shouldn't believe me either--benchmark yourself and then 
you'll know.  As a rule, any time someone suggests there's a 
technological approach that makes it easier to manage disks, that 
approach will also slow performance.  LVM vs. straight volumes, SAN vs. 
direct-attached storage, VM vs. real hardware, it's always the same story.

--
Greg Smith    greg@2ndQuadrant.com    Baltimore, MD