Horton

Re: [Patch] ALTER SYSTEM READ ONLY

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Kapila <amit.kapila16@gmail.com>, amul sul <sulamul@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-06-17T16:34:53Z
Lists: pgsql-hackers 
On Wed, Jun 17, 2020 at 12:27 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Which of the things I mentioned don't require writing WAL?

Writing hint bits and marking index tuples as killed do not write WAL
unless checksums are enabled.

> 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.

See my comments on the nearby pg_demote thread. I think we want both.

> >> 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.

I think that depends a lot on how you view ALTER SYSTEM. I believe it
would be reasonable to view ALTER SYSTEM as a catch-all for commands
that make system-wide state changes, even if those changes are not all
of the same kind as each other; some might be machine-local, and
others cluster-wide; some WAL-logged, and others not. I don't think
it's smart to view ALTER SYSTEM through a lens that boxes it into only
editing postgresql.auto.conf; if that were so, we ought to have called
it ALTER CONFIGURATION FILE or something rather than ALTER SYSTEM. For
that reason, I do not see the choice of syntax as painting us into a
corner.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.