Re: PITR Backups

On Fri, 2007-06-22 at 13:12 -0400, Tom Lane wrote:
> Dan Gorman <dgorman(at)hi5(dot)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