Re: Primary not sending to synchronous standby

On 2015-02-23 17:53:59 +0100, Andres Freund wrote:
> On 2015-02-23 15:48:25 +0000, Thom Brown wrote:
> > On 23 February 2015 at 15:42, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> >
> > > On 2015-02-23 16:38:44 +0100, Andres Freund wrote:
> > > > I unfortunately don't remember enough of the thread to reference it
> > > > here.
> > >
> > > Found the right keywords. The threads below
> > >
> > > http://archives.postgresql.org/message-id/369698E947874884A77849D8FE3680C2%40maumau
> > > and
> > >
> > > http://www.postgresql.org/message-id/5CF4ABBA67674088B3941894E22A0D25@maumau
> > >
> >
> > Yes, this seems to be virtually the same issue reported. The trace looks
> > the same except for RecordTransactionCommit.
>
> So, I proposed in
> http://www.postgresql.org/message-id/20140707155113.GB1136@alap3.anarazel.de
> that we make sequences assign a xid and only wait for syncrep when a xid
> is assigned. The biggest blocker was that somebody would have to do some
> code reviewing to find other locations that might need similar
> treatment.
>
> I did a, quick, grep for XLogInsert() and I think we're otherwise
> fine. There's some debatable cases:
>
> * XLOG_STANDBY_LOCK doesn't force a xid to be assigned. I think it's
> harmless though, as we really only need to wait for that to be
> replicated if the transaction did something relevant (i.e. catalog
> changes). And those will force xid assignment.
> * 2pc records don't assign a xid. But twophase.c does it's own waiting,
> so that's fine.
> * Plain vacuums will not trigger waits. But I think that's good. There's
> really no need to wait if all that's been done is some cleanup without
> visible consequences.
> * Fujii brought up that we might want to wait for XLOG_SWITCH - I don't
> really see why.
> * XLOG_RESTORE_POINT is a similar candidate - I don't see really valid
> arguments for making 2pc wait.
>
>
> The attached, untested, patch changes things so that we
> a) only wait for syncrep if we both wrote WAL and had a xid assigned
> b) use an async commit if we just had a xid assigned, without having
> written WAL, even if synchronous_commit = off
> c) acquire a xid when WAL logging sequence changes (arguable at least
> one of the xid assignments is redundant, but it doesn't cost
> anything, so ...)
>
> I think it makes sense to change a) and b) that way because there's no
> need to wait for WAL flushes/syncrep waits when all that happened is
> manipulations of temporary/unlogged tables or HOT pruning. It's slightly
> wierd that the on-disk flush and the syncrep wait essentially used two
> different mechanisms for deciding when to flush.

I think this patch is a clear improvement - and unless somebody protests
I'm planning to push it sometime not too far away.

What I'm wondering about is what we want to do with the back branches:
It's really rather ugly that enabling syncrep can prevent you from
logging in; especially if the connection that's blocked is the syncrep
walsender. It'll resolve itself if you kill the connections often
enough, but still, that's something you don't wan to have to do. The
patch isn't particularly complex and should only make things efficient,
so it'd be defensible to backpatch it. Thoughts?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services