Re: [Patch] ALTER SYSTEM READ ONLY

On Fri, Jun 26, 2020 at 5:59 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> Any operation working on on-disk relation blocks needs to have a
> consistent state, and a clean shutdown gives this guarantee thanks to
> the shutdown checkpoint (see also pg_rewind). There are two states in
> the control file, shutdown for a primary and shutdown while in
> recovery to cover that. So if you stop the server cleanly but fail to
> see a proper state with pg_checksums, it seems to me that the proposed
> patch does not handle correctly the state of the cluster in the
> control file at shutdown. That's not good.

I think it is actually very good. If a feature that supposedly
prevents writing WAL permitted a shutdown checkpoint to be written, it
would be failing to accomplish its design goal. There is not much of a
use case for a feature that stops WAL from being written except when
it doesn't.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company