* Robert Haas (robertmh...@gmail.com) wrote: > On Wed, Jan 25, 2017 at 7:37 PM, 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. > > 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
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