On Fri, 2007-06-22 at 13:12 -0400, Tom Lane wrote: > Dan Gorman <dgorman@xxxxxxx> 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
