Re: Offline enabling/disabling of data checksums

On Wed, Mar 13, 2019 at 12:24:21PM +0100, Magnus Hagander wrote:
> Enabling or disabling the checksums offline on the master quite clearly
> requires a rebuild of the standby, there is no other way (this is one of
> the reasons for the online enabling in that patch, so I still hope we can
> get that done -- but not for this version).

I am curious to understand why this would require a rebuild of the
standby.  Technically FPWs don't update the checksum of a page when it
is WAL-logged, so even if a primary and a standby don't agree on the
checksum configuration, it is the timing where pages are flushed in
the local instance which counts for checksum correctness.

> You mean if the backend and pg_checksums is built with different blocksize?
> Yeah, that sounds like something which is a cheap check and should be done.

Yes, we should check after that, checksum calculation uses BLCKSZ with
a hardcoded value, so a mismatch would cause computation failures.  It
could be possible to not have this restriction if we made the block
size an argument of the checksum calculation though.
--
Michael