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Subject: Re: PITR Backups
From: "Simon Riggs" <simon@2ndquadrant.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: "Dan Gorman" <dgorman@hi5.com>,
	"Toru SHIMOGAKI" <shimogaki.toru@oss.ntt.co.jp>,
	<pgsql-performance@postgresql.org>
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	<1182509748.9276.178.camel@silverbirch.site>
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	<23875.1182532322@sss.pgh.pa.us>
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Date: Fri, 22 Jun 2007 18:45:57 +0100
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On Fri, 2007-06-22 at 13:12 -0400, Tom Lane wrote:
> Dan Gorman <dgorman@hi5.com> writes:
> > This snapshot is done at the LUN (filer) level, postgres is un-aware  
> > we're creating a backup, so I'm not sure how pg_start_backup() plays  
> > into this ...
> 
> That method works too, as long as you snapshot both the data files and
> WAL files --- when you start PG from the backup, it will think it
> crashed and recover by replaying WAL.  So, assuming that the snapshot
> technology really works, it should be exactly as reliable as crash
> recovery is.  

> 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.

At the very least, you should issue a CHECKPOINT prior to taking the
snapshot, to ensure that the write barriers have gone through.

But that being said, I'm not quite sure why following the Continuous
Archiving procedures is a problem, since they don't add any significant
overhead, over and above the checkpoint command.

-- 
  Simon Riggs             
  EnterpriseDB   http://www.enterprisedb.com