* Robert Haas (robertmhaas@gmail.com) wrote:
> On Wed, Jan 25, 2017 at 7:37 PM, Andres Freund <andres@anarazel.de> wrote:
> > On 2017-01-25 19:30:08 -0500, Stephen Frost wrote:
> >> * Peter Geoghegan (pg@heroku.com) wrote:
> >> > On Wed, Jan 25, 2017 at 3:30 PM, Stephen Frost <sfrost@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.
>
> Yeah. This is also why backups force full page writes on even if
> they're turned off in general.
I've got a question into David about this, I know we chatted about the
risk at one point, I just don't recall what we ended up doing (I can
imagine a few different possible things- re-read the page, which isn't a
guarantee but reduces the chances a fair bit, or check the LSN, or
perhaps the plan was to just check if it's in the WAL, as I mentioned)
or if we ended up concluding it wasn't a risk for some, perhaps
incorrect, reason and need to revisit it.
Thanks!
Stephen