Re: Proposal to add a QNX 6.5 port to PostgreSQL

On 2014-08-10 18:36:18 -0400, Noah Misch wrote:
> [Due for a new subject line?]
>
> On Sat, Aug 09, 2014 at 08:16:01PM +0200, Andres Freund wrote:
> > On 2014-08-09 14:09:36 -0400, Tom Lane wrote:
> > > Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > > > On 2014-08-09 14:00:49 -0400, Tom Lane wrote:
> > > >> I don't think it's anywhere near as black-and-white as you guys claim.
> > > >> What it comes down to is whether allowing existing transactions/sessions
> > > >> to finish is more important than allowing new sessions to start.
> > > >> Depending on the application, either could be more important.
> > >
> > > > Nah. The current behaviour circumvents security measures we normally
> > > > consider absolutely essential. If the postmaster died some bad shit went
> > > > on. The likelihood of hitting corner case bugs where it's important that
> > > > we react to a segfault/panic with a restart/crash replay is rather high.
> > >
> > > What's your point? Once a new postmaster starts, it *will* do a crash
> > > restart, because certainly no shutdown checkpoint ever happened.
> >
> > That's not saying much. For one, there can be online checkpoints in that
> > time. So it's certainly not guaranteed (or even all that likely) that
> > all the WAL since the incident is replayed. For another, it can be
> > *hours* before all the backends finish.
> >
> > IIRC we'll continue to happily write WAL and everything after postmaster
> > (and possibly some backends, corrupting shmem) have crashed. The
> > bgwriter, checkpointer, backends will continue to write dirty buffers to
> > disk. We'll IIRC continue to write checkpoints. That's simply not
> > things we should be doing after postmaster crashed if we can avoid at
> > all.
>
> The basic support processes, including the checkpointer, exit promptly upon
> detecting a postmaster exit. Checkpoints cease.

Only after finishing an 'in process' checkpoint though afaics. And only
if no new checkpoint has been requested since. The latter because we
don't even test for postmaster death if a latch has been set... I think
it's similar for the bgwriter and such.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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