Re: Offline enabling/disabling of data checksums

On Wed, Mar 13, 2019 at 12:40 PM Sergei Kornilov <sk(at)zsrv(dot)org> wrote:

> Hi
>
> >> One new question from me: how about replication?
> >> Case: primary+replica, we shut down primary and enable checksum, and
> "started streaming WAL from primary" without any issue. I have master with
> checksums, but replica without.
> >> Or cluster with checksums, then disable checksums on primary, but
> standby think we have checksums.
> >
> > 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 mean this should be at least documented.
> Change system id... Maybe is reasonable
>

I think this is dangerous enough that it needs to be enforced and not
documented.

Most people who care about checksums are also going to be having either
replication or backup...

>> Also we support ./configure --with-blocksize=(not equals 8)? make check
> on HEAD fails for me. If we support this - i think we need recheck BLCKSZ
> between compiled pg_checksum and used in PGDATA
> >
> > 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.
>
> Yep
>

This one I could more live with it only being a documented problem rather
than enforced, but it also seems very simple to enforce.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>