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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 10:20:09
Message-ID: 1283509209.1834.2902.camel@ebony (view raw or whole thread)
Lists: pgsql-hackers
On Fri, 2010-09-03 at 12:33 +0300, Heikki Linnakangas wrote:
> On 03/09/10 10:45, Simon Riggs wrote:
> > 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.
> Huh? You made a very clear claim above that you don't need additional 
> messages. I explained why I don't think that's true, and asked you to 
> explain why you think it is true. Whether the claim is true or not does 
> not depend on my attitude.

Why exactly would we need to send 3 messages when we could send 1? 
Replace your statements of "it needs to" with "why would it" instead.

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

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Subject: Re: Synchronization levels in SR

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