Re: Checksums by default?

On 2017-01-25 19:30:08 -0500, Stephen Frost wrote:
> * Peter Geoghegan (pg(at)heroku(dot)com) wrote:
> > On Wed, Jan 25, 2017 at 3:30 PM, Stephen Frost <sfrost(at)snowman(dot)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