Re: Configuring synchronous replication

On Fri, 2010-09-17 at 16:09 +0300, Heikki Linnakangas wrote:

> >> I don't expect any meaningful differences in terms of performance
> >> between any of the discussed options. The big question right now is...
> >
> > This is the critical point. Politely, I would observe that *You* do not
> > think there is a meaningful difference. *I* do, and evidence suggests
> > that both Oracle and DRBD think so too. So we differ on what the "big
> > question" is here.
> 
> We must be talking about different things again. There's certainly big 
> differences in the different synchronization levels and configurations, 
> but I don't expect there to be big performance differences between 
> patches to implement those levels. Once we got rid of the polling loops, 
> I expect the network and disk latencies to dominate.

So IIUC you seem to agree with
* 4 levels of synchronous replication (specified on master)
* transaction-controlled replication from the master
* sending 3 LSN values back from standby

Well, then that pretty much is my patch, except for the parameter UI.
Did I misunderstand?

We also agree that we need a standby to master protocol change; I used
Zoltan's directly and I've had zero problems with it in testing.

The only disagreement has been about
* the need for standby registration (I understand "want")
which seems to boil down to whether we wait for servers that *ought* to
be there, but currently aren't.

* whether to have wal writer active (I'm happy to add that later in this
release, so we get the "recv" option also)

* whether we have a parameter for quorum commit > 1 (happy to add later)
Not sure if there is debate about whether quorum_commit = 1 is the
default.

* whether we provide replication_exceptions as core feature or as a
plugin

The only area of doubt is when we send replies, which you haven't
thought about yet. So presumably you've no design-level objection to
what I've proposed.

Things we all seem to like are

* different standbys can offer different sync levels

* standby names

* a set returning function which tells you current LSNs of all standbys

* the rough idea of being able to specify a "service" and have that
equate to a more complex config underneath the covers, without needing
to have the application know the details - I think we need more details
on that before we could say "we agree".

So seems like a good days work.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services