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Re: Synchronous replication - patch status inquiry

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Fetter <david(at)fetter(dot)org>, Bruce Momjian <bruce(at)momjian(dot)us>, fazool mein <fazoolmein(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Synchronous replication - patch status inquiry
Date: 2010-09-03 06:36:27
Message-ID: 1283495787.1834.2624.camel@ebony (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, 2010-09-03 at 12:50 +0900, Fujii Masao wrote:
> On Thu, Sep 2, 2010 at 11:32 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> > I understand what you're after, the idea of being able to set
> > synchronization level on a per-transaction basis is cool. But I haven't seen
> > a satisfactory design for it. I don't understand how it would work in
> > practice. Even though it's cool, having different kinds of standbys
> > connected is a more common scenario, and the design needs to accommodate
> > that too. I'm all ears if you can sketch a design that can do that.
> That design would affect what the standby should reply. If we choose
> async/recv/fsync/replay on a per-transaction basis, the standby
> should send multiple LSNs and the master needs to decide when
> replication has been completed. OTOH, if we choose just sync/async,
> the standby has only to send one LSN.
> The former seems to be more useful, but triples the number of ACK
> from the standby. I'm not sure whether its overhead is ignorable,
> especially when the distance between the master and the standby is
> very long.

No, it doesn't. There is no requirement for additional messages. It just
adds 16 bytes onto the reply message, maybe 24. If there is a noticeable
overhead from that, shoot me. 

 Simon Riggs 
 PostgreSQL Development, 24x7 Support, Training and Services

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