Subject: Re: [GENERAL] Backup
From: Simon Riggs <simon@2ndquadrant.com>
To: Chander Ganesan <chander@otg-nc.com>
Cc: Magnus Hagander <magnus@hagander.net>,
	Phillip Smith <phillip.smith@weatherbeeta.com.au>,
	"'Dominic Carlucci'" <carlucci@cae.com>, pgsql-general@postgresql.org,
	pgsql-admin@postgresql.org, carlucci@cae.ca
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	<20080131143405.GC21660@svana.org>
	<20080131144314.GH11977@svr2.hagander.net>
	<47A1E2F6.7060108@otg-nc.com>
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Date: Thu, 31 Jan 2008 15:58:32 +0000
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On Thu, 2008-01-31 at 10:02 -0500, Chander Ganesan wrote:
> Magnus Hagander wrote:
> > On Thu, Jan 31, 2008 at 03:34:05PM +0100, Martijn van Oosterhout wrote:
> >   
> >> On Thu, Jan 31, 2008 at 01:28:48PM +0000, Simon Riggs wrote:
> >>     
> >>> That sentence has no place in any discussion about "backup" because the
> >>> risk is not just a few transactions, it is a corrupt and inconsistent
> >>> database from which both old and new data would be inaccessible.
> >>>       
> >> Hmm? I thought the whole point of a filesystem snapshot was that it's
> >> the same as if the system crashed. And I was fairly sure we could
> >> recover from that...
> >>     
> >
> > That was my assumption as well. *Assuming* that the filesystem snapshot is
> > consistent. There are a bunch of solutions that don't do consistent
> > snapshots between different partitions, so if your WAL or one tablespace is
> > on a different partition, you'll get corruption anyway... (seen this in
> > Big Commercial Database, so that's not a pg problem)
> >   
> Agreed.  That's why I made it a point to mention that all of your 
> tablespaces should be on the same file system...  In hindsight, I should 
> have also stated that your WAL logs should be on the same file system as 
> well...

I think we all understand and agree, I just start twitching when anyone
talks about it being OK to lose transactions when backing up. You meant
the ones currently in progress, not the ones already committed and on
disk.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com