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Re: Synchronous replication - patch status inquiry

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(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 06:55:37
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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.

   Heikki Linnakangas

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