From: Simon Riggs <simon@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dan Gorman <dgorman@hi5.com>
Cc: Toru SHIMOGAKI <shimogaki.toru@oss.ntt.co.jp>
Cc: pgsql-performance@postgresql.org
Subject: Re: PITR Backups
Date: Fri, 22 Jun 2007 18:45:57 +0100
Message-ID: <1182534358.9276.326.camel@silverbirch.site> (raw)
In-Reply-To: <23875.1182532322@sss.pgh.pa.us>
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>
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
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To: pgsql-performance@postgresql.org
Cc: simon@2ndquadrant.com, tgl@sss.pgh.pa.us, dgorman@hi5.com, shimogaki.toru@oss.ntt.co.jp
Subject: Re: PITR Backups
In-Reply-To: <1182534358.9276.326.camel@silverbirch.site>
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