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Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: 2011-03-18 15:40:57
Message-ID: 1300462857.18619.14696.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
On Fri, 2011-03-18 at 11:07 -0400, Robert Haas wrote:
> On Fri, Mar 18, 2011 at 10:55 AM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> > On Thu, Mar 17, 2011 at 5:46 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >> What makes more sense to me after having thought about this more
> >> carefully is to simply make a blanket rule that when
> >> synchronous_replication=on, synchronous_commit has no effect.  That is
> >> easy to understand and document.
> >
> > For what it's worth "has no effect" doesn't make much sense to me.
> > It's a boolean, either commits are going to block or they're not.
> >
> > What happened to the idea of a three-way switch?
> >
> > synchronous_commit = off
> > synchronous_commit = disk
> > synchronous_commit = replica
> >
> > With "on" being a synonym for "disk" for backwards compatibility.
> >
> > Then we could add more options later for more complex conditions like
> > waiting for one server in each data centre or waiting for one of a
> > certain set of servers ignoring the less reliable mirrors, etc.
> 
> This is similar to what I suggested upthread, except that I suggested
> on/local/off, with the default being on.  That way if you set
> synchronous_standby_names, you get synchronous replication without
> changing another setting, but you can say local instead if for some
> reason you want the middle behavior.  If we're going to do it all with
> one GUC, I think that way makes more sense.  If you're running sync
> rep, you might still have some transactions that you don't care about,
> but that's what async commit is for.  It's a funny kind of transaction
> that we're OK with losing if we have a failover but we're not OK with
> losing if we have a local crash from which we recover without failing
> over.

I much prefer a single switch, which is what I originally suggested.
Changing the meaning of synchronous_commit seems a problem.

durability = localmemory
durability = localdisk
(durability = remotereceive - has no meaning in current code)
durability = remotedisk
durability = remoteapply

it also allows us to have in the future

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services
 


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