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Re: Synchronous replication

From: Joshua Tolley <eggyknap(at)gmail(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Yeb Havinga <yebhavinga(at)gmail(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>,Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Synchronous replication
Date: 2010-07-27 14:57:03
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Lists: pgsql-hackers
On Tue, Jul 27, 2010 at 10:53:45PM +0900, Fujii Masao wrote:
> On Tue, Jul 27, 2010 at 10:12 PM, Joshua Tolley <eggyknap(at)gmail(dot)com> wrote:
> > My concern is that in a quorum system, if the quorum number is less than the
> > total number of replicas, there's no way to know *which* replicas composed the
> > quorum for any given transaction, so we can't know which servers to fail to if
> > the master dies.
> What about checking the current WAL receive location of each standby by
> using pg_last_xlog_receive_location()? The standby which has the newest
> location should be failed over to.

That makes sense. Thanks.

> > This isn't different from Oracle, where it looks like
> > essentially the "quorum" value is always 1. Your scenario shows that all
> > replicas are not created equal, and that sometimes we'll be interested in WAL
> > getting committed on a specific subset of the available servers. If I had two
> > nearby replicas called X and Y, and one at a remote site called Z, for
> > instance, I'd set quorum to 2, but really I'd want to say "wait for server X
> > and Y before committing, but don't worry about Z".
> >
> > I have no idea how to set up our GUCs to encode a situation like that :)
> Yeah, quorum commit alone cannot cover that situation. I think that
> current approach (i.e., quorum commit plus replication mode per standby)
> would cover that. In your example, you can choose "recv", "fsync" or
> "replay" as replication_mode in X and Y, and choose "async" in Z.

Clearly I need to read through the GUCs and docs better. I'll try to keep
quiet until that's finished :)

Joshua Tolley / eggyknap
End Point Corporation

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