Message-ID: <52A99310.6010301@lab.ntt.co.jp>
Date: Thu, 12 Dec 2013 19:42:24 +0900
From: KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.jp>
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To: Simon Riggs <simon@2ndQuadrant.com>
CC: Pavel Stehule <pavel.stehule@gmail.com>, =?UTF-8?B?RmFicsOtemlvIE1lbGw=?=
 =?UTF-8?B?bw==?= <fabriziomello@gmail.com>,
 Robert Haas <robertmhaas@gmail.com>,
 Pgsql Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Time-Delayed Standbys
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(2013/12/12 18:09), Simon Riggs wrote:
> On 9 December 2013 10:54, KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.jp> wrote:
>> (2013/12/09 19:35), Pavel Stehule wrote:
>>>
>>>
>>>
>>>
>>> 2013/12/9 KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.jp
>>> <mailto:kondo.mitsumasa@lab.ntt.co.jp>>
>>>
>>>
>>>      Hi Fabrízio,
>>>
>>>      I test your v4 patch, and send your review comments.
>>>
>>>      * Fix typo
>>>       > 49 -# commited transactions from the master, specify a recovery
>>> time delay.
>>>       > 49 +# committed transactions from the master, specify a recovery
>>> time delay.
>>>
>>>      * Fix white space
>>>       > 177 -               if (secs <= 0 && microsecs <=0)
>>>       > 177 +               if (secs <= 0 && microsecs <=0 )
>>>
>>>      * Add functionality (I propose)
>>>      We can set negative number at min_standby_apply_delay. I think that
>>> this feature
>>>      is for world wide replication situation. For example, master server is
>>> in
>>>      Japan and slave server is in San Francisco. Japan time fowards than
>>> San
>>>      Francisco time
>>>      . And if we want to delay in this situation, it can need negative
>>> number in
>>>      min_standby_apply_delay. So I propose that time delay conditional
>>> branch
>>>      change under following.
>>>       > - if (min_standby_apply_delay > 0)
>>>       > + if (min_standby_apply_delay != 0)
>>>      What do you think? It might also be working collectry.
>>>
>>>
>>> what using interval instead absolute time?
>>
>> This is because local time is recorded in XLOG. And it has big cost for
>> calculating global time.
>
> I agree with your request here, but I don't think negative values are
> the right way to implement that, at least it would not be very usable.
I think that my proposal is the easiest and simplist way to solve this problem. 
And I believe that the man who cannot calculate the difference in time-zone 
doesn't set replication cluster across continents.

> My suggestion would be to add the TZ to the checkpoint record. This
> way all users of WAL can see the TZ of the master and act accordingly.
> I'll do a separate patch for that.
It is something useful for also other situations. However, it might be
going to happen long and complicated discussions... I　think that our hope is to 
commit this patch in this commit-fest or next final commit-fest.

Regards,
--
Mitsumasa KONDO
NTT Open Source Software Center


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