Re: [Patch] ALTER SYSTEM READ ONLY

On Thu, Jan 28, 2021 at 7:17 AM Amul Sul <sulamul(at)gmail(dot)com> wrote:
> I am still on this. The things that worried me here are the wal records sequence
> being written in the startup process -- UpdateFullPageWrites() generate record
> just before the recovery check-point record and XLogReportParameters() just
> after that but before any other backend could write any wal record. We might
> also need to follow the same sequence while changing the system to read-write.

I was able to chat with Andres about this topic for a while today and
he made some proposals which seemed pretty good to me. I can't promise
that what I'm about to write is an entirely faithful representation of
what he said, but hopefully it's not so far off that he gets mad at me
or something.

1. If the server starts up and is read-only and
ArchiveRecoveryRequested, clear the read-only state in memory and also
in the control file, log a message saying that this has been done, and
proceed. This makes some other cases simpler to deal with.

2. Create a new function with a name like XLogAcceptWrites(). Move the
following things from StartupXLOG() into that function: (1) the call
to UpdateFullPageWrites(), (2) the following block of code that does
either CreateEndOfRecoveryRecord() or RequestCheckpoint() or
CreateCheckPoint(), (3) the next block of code that runs
recovery_end_command, (4) the call to XLogReportParameters(), and (5)
the call to CompleteCommitTsInitialization(). Call the new function
from the place where we now call XLogReportParameters(). This would
mean that (1)-(3) happen later than they do now, which might require
some adjustments.

3. If the system is starting up read only (and the read-only state
didn't get cleared because of #1 above) then don't call
XLogAcceptWrites() at the end of StartupXLOG() and instead have the
checkpointer do it later when the system is going read-write for the
first time.

--
Robert Haas
EDB: http://www.enterprisedb.com