Re: [Patch] ALTER SYSTEM READ ONLY

On Tue, 11 May 2021 at 7:50 PM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:

> On Tue, May 11, 2021 at 6:56 PM Amul Sul <sulamul(at)gmail(dot)com> wrote:
> >
> > On Tue, May 11, 2021 at 6:48 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com>
> wrote:
> > >
> > > On Tue, May 11, 2021 at 4:50 PM Amul Sul <sulamul(at)gmail(dot)com> wrote:
> > > >
> > > > On Tue, May 11, 2021 at 4:13 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com>
> wrote:
> > > > > I might be missing something, but assume the behavior should be
> like this
> > > > >
> > > > > 1. If the state is getting changed from
> WALPROHIBIT_STATE_READ_WRITE
> > > > > -> WALPROHIBIT_STATE_READ_ONLY, then as soon as the backend process
> > > > > the barrier, we can immediately abort any read-write
> transaction(and
> > > > > stop allowing WAL writing), because once we ensure that all session
> > > > > has responded that now they have no read-write transaction then we
> can
> > > > > safely change the state from WALPROHIBIT_STATE_GOING_READ_ONLY to
> > > > > WALPROHIBIT_STATE_READ_ONLY.
> > > > >
> > > >
> > > > Yes, that's what the current patch is doing from the first patch
> version.
> > > >
> > > > > 2. OTOH, if we are changing from WALPROHIBIT_STATE_READ_ONLY ->
> > > > > WALPROHIBIT_STATE_READ_WRITE, then we don't need to allow the
> backend
> > > > > to consider the system as read-write, instead, we should wait until
> > > > > the shared state is changed to WALPROHIBIT_STATE_READ_WRITE.
> > > > >
> > > >
> > > > I am sure that only not enough will have the same issue where
> > > > LocalXLogInsertAllowed gets set the same as the read-only as
> described in
> > > > my previous reply.
> > >
> > > Okay, but while browsing the code I do not see any direct if condition
> > > based on the "LocalXLogInsertAllowed" variable, can you point me to
> > > some references?
> > > I only see one if check on this variable and that is in
> > > XLogInsertAllowed() function, but now in XLogInsertAllowed() function,
> > > you are already checking IsWALProhibited. No?
> > >
> >
> > I am not sure I understood this. Where am I checking IsWALProhibited()?
> >
> > IsWALProhibited() is called by XLogInsertAllowed() once when
> > LocalXLogInsertAllowed is in a reset state, and that result will be
> > cached in LocalXLogInsertAllowed and will be used in the subsequent
> > XLogInsertAllowed() call.
>
> Okay, got what you were trying to say. But that can be easily
> fixable, I mean if the state is WALPROHIBIT_STATE_GOING_READ_WRITE
> then what we can do is don't allow to write the WAL but let's not set
> the LocalXLogInsertAllowed to 0. So until we are in the intermediate
> state WALPROHIBIT_STATE_GOING_READ_WRITE, we will always have to rely
> on GetWALProhibitState(), I know this will add a performance penalty
> but this is for the short period until we are in the intermediate
> state. After that as soon as it will set to
> WALPROHIBIT_STATE_READ_WRITE then the XLogInsertAllowed() will set
> LocalXLogInsertAllowed to 1.

I think I have much easier solution than this, will post that with update
version patch set tomorrow.

Regards,
Amul