Re: [Patch] ALTER SYSTEM READ ONLY

On Fri, Jul 24, 2020 at 7:34 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

>
> On Thu, Jul 23, 2020 at 12:11 PM Soumyadeep Chakraborty
> <soumyadeep2007(at)gmail(dot)com> wrote:
> > In the read-only level I was suggesting, I wasn't suggesting that we
> > stop WAL flushes, in fact we should flush the WAL before we mark the
> > system as read-only. Once the system declares itself as read-only, it
> > will not perform any more on-disk changes; It may perform all the
> > flushes it needs as a part of the read-only request handling.
>
> I think that's already how the patch works, or at least how it should
> work. You stop new writes, flush any existing WAL, and then declare
> the system read-only. That can all be done quickly.
>

True, except for the fact that it allows dirty buffers to be flushed
after the ALTER command returns.

> > What I am saying is it doesn't have to be just the queries. I think we
> > can cater to all the other use cases simply by forcing a checkpoint
> > before marking the system as read-only.
>
> But that part can't, which means that if we did that, it would break
> the feature for the originally intended use case. I'm not on board
> with that.
>

Referring to the options you presented in [1]:
I am saying that we should allow for both: with a checkpoint (#2) (can
also be #3) and without a checkpoint (#1) before having the ALTER
command return, by having different levels of read-onlyness.

We should have syntax variants for these. The syntax should not be an
ALTER SYSTEM SET as you have pointed out before. Perhaps:

ALTER SYSTEM READ ONLY; -- #2 or #3
ALTER SYSTEM READ ONLY WAL; -- #1
ALTER SYSTEM READ WRITE;

or even:

ALTER SYSTEM FREEZE; -- #2 or #3
ALTER SYSTEM FREEZE WAL; -- #1
ALTER SYSTEM UNFREEZE;

Regards,
Soumyadeep (VMware)

[1] http://postgr.es/m/CA+TgmoZ-c3Dz9QwHwmm4bc36N4u0XZ2OyENewMf+BwokbYdK9Q@mail.gmail.com