On Wed, 16 Jan 2008 10:19:12 -0500 Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Steve Holdoway <steve.holdoway@xxxxxxxxxxxxx> writes: > > You can be absolutely certain that the tar backup of a file that's changed is a complete waste of time. Because it changed while you were copying it. > > That is, no doubt, the reasoning that prompted the gnu tar people to > make it do what it does, but it has zero to do with reality for > Postgres' usage in PITR base backups. What we care about is consistency > on the page level: as long as each page of the backed-up file correctly > represents *some* state of that page while the backup was in progress, > everything is okay, because replay of the WAL log will correct any pages > that are out-of-date, missing, or shouldn't be there at all. And > Postgres always writes whole pages. So as long as write() and read() > are atomic --- which is the case on all Unixen I know of --- everything > works. > > (Thinks for a bit...) Actually I guess there's one extra assumption in > there, which is that tar must issue its reads in multiples of our page > size. But that doesn't seem like much of a stretch. > > regards, tom lane That's OK for the WAL logs, but what about the initial archive - the recovery's got to start somewhere...
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