Re: Synchronous replication - patch status inquiry

On Fri, 2010-09-03 at 12:50 +0900, Fujii Masao wrote:
> On Thu, Sep 2, 2010 at 11:32 PM, Heikki Linnakangas
> <heikki.linnakangas@enterprisedb.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           www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services