Re: Proposal to add a QNX 6.5 port to PostgreSQL

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
> only issue here is what grace period existing orphaned backends are given
> to finish their work --- and it's not possible for the answer to that
> to be "zero", so you don't get to assume that nothing happens in
> backend-land after the instant of postmaster crash.

Sure. But I don't think a window in the range of seconds comes close to
being the same as a window that easily can be hours.

Greetings,

Andres Freund

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