Horton

Re: [Patch] ALTER SYSTEM READ ONLY

Tom Lane <tgl@sss.pgh.pa.us>

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, amul sul <sulamul@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-06-17T16:27:48Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> This seems like pretty dubious hand-waving. Of course, things that
> write WAL are going to be broken by a switch that prevents writing
> WAL; but if they were not, there would be no purpose in having such a
> switch, so that's not really an argument. But you seem to have mixed
> in some things that don't require writing WAL, and claimed without
> evidence that those would somehow also be broken.

Which of the things I mentioned don't require writing WAL?

You're right that these are the same things that we already forbid on a
standby, for the same reason, so maybe it won't be as hard to identify
them as I feared.  I wonder whether we should envision this as "demote
primary to standby" rather than an independent feature.

>> I also think that putting such a thing into ALTER SYSTEM has got big
>> logical problems.

> ... no right-thinking person would ever propose to
> change a feature that renders the system read-only in such a way that
> it was impossible to deactivate it. That would be nuts.

My point was that putting this in ALTER SYSTEM paints us into a corner
as to what we can do with ALTER SYSTEM in the future: we won't ever be
able to make that do anything that would require writing WAL.  And I
don't entirely believe your argument that that will never be something
we'd want to do.

			regards, tom lane

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.