Re: [Patch] ALTER SYSTEM READ ONLY

On Wed, Feb 17, 2021 at 7:50 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2021-02-16 17:11:06 -0500, Robert Haas wrote:

Thank you very much to both of you !

> > I can't promise that what I'm about to write is an entirely faithful
> > representation of what he said, but hopefully it's not so far off that
> > he gets mad at me or something.
>
> Seems accurate - and also I'm way too tired that I'd be mad ;)
>
>
> > 1. If the server starts up and is read-only and
> > ArchiveRecoveryRequested, clear the read-only state in memory and also
> > in the control file, log a message saying that this has been done, and
> > proceed. This makes some other cases simpler to deal with.
>
> It seems also to make sense from a behaviour POV to me: Imagine a
> "smooth" planned failover with ASRO:
> 1) ASRO on primary
> 2) promote standby
> 3) edit primary config to include primary_conninfo, add standby.signal
> 4) restart "read only primary"
>
> There's not really any spot in which it'd be useful to do disable ASRO,
> right? But 4) should make the node a normal standby.
>

Understood.

In the attached version I have made the changes accordingly what Robert has
summarised in his previous mail[1].

In addition to that, I also move the code that updates the control file to
XLogAcceptWrites() which will also get skipped when the system is read-only (wal
prohibited). The system will be in the crash recovery, and that will
change once we do the end-of-recovery checkpoint and the WAL writes operation
which we were skipping from startup. The benefit of keeping the system in
recovery mode is that it fixes my concern[2] where other backends could connect
and write wal records while we were changing the system to read-write. Now, no
other backends allow a wal write; UpdateFullPageWrites(), end-of-recovery
checkpoint, and XLogReportParameters() operations will be performed in the same
sequence as it is in the startup while changing the system to read-write.

Regards,
Amul

1] http://postgr.es/m/CA+TgmoZ=CCTbAXxMTYZoGXEgqzOz9smkBWrDpsacpjvFcGCuaw@mail.gmail.com
2] http://postgr.es/m/CAAJ_b97xX-nqRyM_uXzecpH9aSgoMROrDNhrg1N51fDCDwoy2g@mail.gmail.com