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Re: Replication server timeout patch

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Replication server timeout patch
Date: 2011-03-11 13:18:22
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Lists: pgsql-hackers
On Fri, Mar 11, 2011 at 8:14 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Mon, Mar 7, 2011 at 8:47 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> On Sun, Mar 6, 2011 at 11:10 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>>> On Sun, Mar 6, 2011 at 5:03 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>>>>> Why does internal_flush_if_writable compute bufptr differently from
>>>>> internal_flush?  And shouldn't it be static?
>>>>> It seems to me that this ought to be refactored so that you don't
>>>>> duplicate so much code.  Maybe static int internal_flush(bool
>>>>> nonblocking).
>>>>> I don't think that the while (bufptr < bufend) loop needs to contain
>>>>> the code to set and clear the nonblocking state.  You could do the
>>>>> whole loop with nonblocking mode turned on and then reenable it just
>>>>> once at the end.  Besides possibly being clearer, that would be more
>>>>> efficient and leave less room for unexpected failures.
>>>> All these comments seem to make sense. Will fix. Thanks!
>>> Done. I attached the updated patch.
>> I rebased the patch against current git master.
> I added this replication timeout patch into next CF.
> I explain why this feature is required for the future review;
> Without this feature, walsender might unexpectedly remain for a while when
> the standby crashes or the network outage happens. TCP keepalive can
> improve this situation to a certain extent, but it's not perfect. Remaining
> walsender can cause some problems.
> For example, when hot_standby_feedback is enabled, such a remaining
> walsender would prevent oldest xmin from advancing and interfere with
> vacuuming on the master. For example, when you use synchronous
> replication and walsender in SYNC mode gets stuck, any synchronous
> standby candidate cannot switch to SYNC mode until that walsender exits,
> and all the transactions would pause.
> This feature causes walsender to exit when there is no reply from the
> standby before the replication timeout expires. Then we can avoid the
> above problems.

I think we should consider making this change for 9.1.  This is a real
wart, and it's going to become even more of a problem with sync rep, I

Robert Haas
The Enterprise PostgreSQL Company

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