Re: [patch] demote

On Tue, 18 Aug 2020 17:41:31 +0200
Jehan-Guillaume de Rorthais <jgdr@dalibo.com> wrote:

> Hi,
> 
> Please find in attachment v5 of the patch set rebased on master after various
> conflicts.
> 
> Regards,
> 
> On Wed, 5 Aug 2020 00:04:53 +0200
> Jehan-Guillaume de Rorthais <jgdr@dalibo.com> wrote:
> 
> > Demote now keeps backends with no active xid alive. Smart mode keeps all
> > backends: it waits for them to finish their xact and enter read-only. Fast
> > mode terminate backends wit an active xid and keeps all other ones.
> > Backends enters "read-only" using LocalXLogInsertAllowed=0 and flip it to -1
> > (check recovery state) once demoted.
> > During demote, no new session is allowed.
> > 
> > As backends with no active xid survive, a new SQL admin function
> > "pg_demote(fast bool, wait bool, wait_seconds int)" had been added.

Just to keep the list inform, I found a race condition leading to backends
trying to write to XLog after they processed the demote signal. Eg.:

  [posmaster]      LOG:  all backends in read only
  [checkpointer]   LOG:  demoting
  [backend]      PANIC:  cannot make new WAL entries during recovery
             STATEMENT:  UPDATE pgbench_accounts [...]

Because of this Postmaster enters in crash recovery while demote
environnement is in progress.

I have a couple of other subjects right now, but I plan to get back to it soon.

Regards,