Horton

Re: [Patch] ALTER SYSTEM READ ONLY

Amul Sul <sulamul@gmail.com>

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From: amul sul <sulamul@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: tushar <tushar.ahuja@enterprisedb.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-06-18T04:19:45Z
Lists: pgsql-hackers 
On Wed, Jun 17, 2020 at 8:15 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Wed, Jun 17, 2020 at 9:51 AM tushar <tushar.ahuja@enterprisedb.com> wrote:
> > 1) ALTER SYSTEM
> >
> > postgres=# alter system read only;
> > ALTER SYSTEM
> > postgres=# alter  system reset all;
> > ALTER SYSTEM
> > postgres=# create table t1(n int);
> > ERROR:  cannot execute CREATE TABLE in a read-only transaction
> >
> > Initially i thought after firing 'Alter system reset all' , it will be
> > back to  normal.
> >
> > can't we have a syntax like - "Alter system set read_only='True' ; "
>
> No, this needs to be separate from the GUC-modification syntax, I
> think. It's a different kind of state change. It doesn't, and can't,
> just edit postgresql.auto.conf.
>
> > 2)When i connected to postgres in a single user mode , i was not able to
> > set the system in read only
> >
> > [edb@tushar-ldap-docker bin]$ ./postgres --single -D data postgres
> >
> > PostgreSQL stand-alone backend 14devel
> > backend> alter system read only;
> > ERROR:  checkpointer is not running
> >
> > backend>
>
> Hmm, that's an interesting finding. I wonder what happens if you make
> the system read only, shut it down, and then restart it in single-user
> mode. Given what you see here, I bet you can't put it back into a
> read-write state from single user mode either, which seems like a
> problem. Either single-user mode should allow changing between R/O and
> R/W, or alternatively single-user mode should ignore ALTER SYSTEM READ
> ONLY and always allow writes anyway.
>
Ok, will try to enable changing between R/O and R/W in the next version.

Thanks Tushar for the testing.

Regards,
Amul

Commits

  1. Initialize variable to placate compiler.

  2. StartupXLOG: Don't repeatedly disable/enable local xlog insertion.

  3. StartupXLOG: Call CleanupAfterArchiveRecovery after XLogReportParameters.

  4. Postpone some end-of-recovery operations related to allowing WAL.

  5. Refactor some end-of-recovery code out of StartupXLOG().

  6. Re-enable contrib/bloom's TAP tests.

  7. Remove unnecessary call to ReadCheckpointRecord().

  8. Allow for error or refusal while absorbing a ProcSignalBarrier.

  9. Add comment to explain an unused function parameter

  10. Extend the ProcSignal mechanism to support barriers.

  11. At promotion, don't leave behind a partial segment on the old timeline.