On Sat, Mar 3, 2018 at 7:32 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Fri, Mar 2, 2018 at 6:26 PM, Tomas Vondra > <tomas.von...@2ndquadrant.com> wrote: >> Hmmm, OK. So we need to have a valid checksum on a page, disable >> checksums, set some hint bits on the page (which won't be WAL-logged), >> enable checksums again and still get a valid checksum even with the new >> hint bits? That's possible, albeit unlikely. > > No, the problem is if - as is much more likely - the checksum is not > still valid.
Hmm, on second thought ... maybe I didn't think this through carefully enough. If the checksum matches on the master by chance, and the page is the same on the standby, then we're fine, right? It's a weird accident, but nothing is actually broken. The failure scenario is where the standby has a version of the page with a bad checksum, but the master has a good checksum. So for example: checksums disabled, master modifies the page (which is replicated), master sets some hint bits (coincidentally making the checksum match), now we try to turn checksums on and don't re-replicate the page because the checksum already looks correct. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
