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). > This isn't like a case where only half the page made it to the disk > because of a system failure though; everything is online and working > properly during an online backup. I don't think that really changes anything. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers