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Re: WalSndWakeup() and synchronous_commit=off

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: WalSndWakeup() and synchronous_commit=off
Date: 2012-05-30 23:33:33
Message-ID: CAHGQGwFx5w_egGt_G6uiV43UAYV3QjFeLaubKnX=2scuThYtjQ@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, May 30, 2012 at 9:46 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On Tuesday, May 29, 2012 08:42:43 PM Andres Freund wrote:
>> Patch attached.
> Imo this patch should be backported to 9.1, 9.0 doesn't use latches and does
> not do explicit wakeup of the sender so its not applicable there.
>
> I can prepare a patch for 9.1 if people agree, there has been some amount of
> change that won't make it apply cleanly.

The patch wakes up walsender more frequently than now. Which leads
walsender to send smaller WAL data packet more frequently, and furthermore
which leads walreceiver to issue fsync more frequently. So I'm afraid that
the patch makes the disk more busy and slows down the read-only query
in the standby. I'm also afraid that frequent fsync calls degrade the
performance
of sync replication. So it's better to do benchmark to address the concerns.

Regards,

-- 
Fujii Masao

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