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

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: 2011-03-18 15:07:23
Message-ID: AANLkTikmbNW=phsUiXJMLv7AaptObRL-pCea-Si-T=ms@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
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.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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