To: "Simon Riggs" <simon@2ndquadrant.com>
cc: "Dan Gorman" <dgorman@hi5.com>,
	"Toru SHIMOGAKI" <shimogaki.toru@oss.ntt.co.jp>,
	pgsql-performance@postgresql.org
Subject: Re: PITR Backups 
In-reply-to: <1182534358.9276.326.camel@silverbirch.site> 
References: <7E0807EE-B970-4869-A248-B838E42AD9EE@hi5.com>
	<9761.1182472005@sss.pgh.pa.us> <467B3459.9030808@oss.ntt.co.jp>
	<1182509748.9276.178.camel@silverbirch.site>
	<31812EEE-22E7-41CD-B70F-CE861681BABC@hi5.com>
	<23875.1182532322@sss.pgh.pa.us>
	<1182534358.9276.326.camel@silverbirch.site>
Comments: In-reply-to "Simon Riggs" <simon@2ndquadrant.com>
	message dated "Fri, 22 Jun 2007 18:45:57 +0100"
Date: Fri, 22 Jun 2007 14:02:44 -0400
Message-ID: <24724.1182535364@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Simon Riggs" <simon@2ndquadrant.com> writes:
> On Fri, 2007-06-22 at 13:12 -0400, Tom Lane wrote:
>> If you saw a problem I'd be inclined to question whether
>> there is some upstream component (OS or disk controller) that's
>> reordering writes.

> Given thats exactly what they do, constantly, I don't think its safe to
> say that it works since we cannot verify whether that has happened or
> not.

If he's trying to snapshot at a level of hardware that's behind a
write-caching disk controller, I agree that that's untrustworthy.

If not, ie if he's snapshotting the actual durable state of the storage
system, then any problems in the snapshot indicate a problem with the
database's ability to recover from a crash.  So I don't think you should
tell him to not worry.

			regards, tom lane