* Andres Freund (and...@anarazel.de) wrote: > On 2017-01-25 19:30:08 -0500, Stephen Frost wrote: > > * Peter Geoghegan (p...@heroku.com) wrote: > > > On Wed, Jan 25, 2017 at 3:30 PM, Stephen Frost <sfr...@snowman.net> wrote: > > > > As it is, there are backup solutions which *do* check the checksum when > > > > backing up PG. This is no longer, thankfully, some hypothetical thing, > > > > but something which really exists and will hopefully keep users from > > > > losing data. > > > > > > Wouldn't that have issues with torn pages? > > > > No, why would it? The page has either been written out by PG to the OS, > > in which case the backup s/w will see the new page, or it hasn't been. > > Uh. Writes aren't atomic on that granularity. That means you very well > *can* see a torn page (in linux you can e.g. on 4KB os page boundaries > of a 8KB postgres page). Just read a page while it's being written out. > > You simply can't reliably verify checksums without replaying WAL (or > creating a manual version of replay, as in checking the WAL for a FPW).
Looking through the WAL isn't any surprise and is something we've been planning to do for other reasons anyway. Thanks! Stephen
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