Re: Synchronous replication - patch status inquiry

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
EnterpriseDB http://www.enterprisedb.com