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Re: Synchronous replication - patch status inquiry

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(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 07:45:30
Message-ID: 1283499930.1834.2711.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, 2010-09-03 at 09:55 +0300, Heikki Linnakangas wrote:
> On 03/09/10 09:36, Simon Riggs wrote:
> > On Fri, 2010-09-03 at 12:50 +0900, Fujii Masao wrote:
> >> 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.
> 
> Please explain how you do it then. When a commit record is sent to the 
> standby, it needs to acknowledge it 1) when it has received it, 2) when 
> it fsyncs it to disk and c) when it's replayed. I don't see how you can 
> get around that.
> 
> Perhaps you can save a bit by combining multiple messages together, like 
> in Nagle's algorithm, but then you introduce extra delays which is 
> exactly what you don't want.

>From my perspective, you seem to be struggling to find reasons why this
should not happen, rather than seeing the alternatives that would
obviously present themselves if your attitude was a positive one. We
won't make any progress with this style of discussion.

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


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