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Re: Synchronization levels in SR

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Synchronization levels in SR
Date: 2010-05-27 10:21:13
Message-ID: AANLkTinb4pc9YaKbJ93fCLxEJ9UW9iphhdhgk1Vk_llD@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, May 27, 2010 at 6:30 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Who sends the ack message?

walreceiver

> Who receives it?

walsender

> Would it be easier to have
> this happen in a second pair of processes WALSynchroniser (on primary)
> and WAL Acknowledger (on standby). WALAcknowledger would send back a
> stream of ack messages with latest xlog positions. WALSynchroniser would
> receive these messages and wake up sleeping backends. If we did that
> then there'd be almost no change at all to existing code, just
> additional code and processes for the sync case. Code would be separate
> and there would be no performance concerns either.

No, this seems to be bad idea. We should not establish extra connection
between servers. That would be a source of trouble.

> If you do choose to make #3 important, then I'd say you need to work out
> how to make WALWriter active as well, so it can perform regular fsyncs,
> rather than having WALReceiver wait across that I/O.

Yeah, this might be an option for optimization though I'm not sure how
it has good effect.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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