Message-ID: <55C262DD.6050402@2ndquadrant.com>
Date: Wed, 05 Aug 2015 21:24:13 +0200
From: Petr Jelinek <petr@2ndquadrant.com>
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To: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Robert Haas <robertmhaas@gmail.com>
CC: Fujii Masao <masao.fujii@gmail.com>,
 pgsql-docs <pgsql-docs@postgresql.org>,
 Pg Hackers <pgsql-hackers@postgresql.org>,
 Craig Ringer <craig@2ndquadrant.com>
Subject: Re: max_worker_processes on the standby
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 <20150804044126.GO2441@postgresql.org>
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On 2015-08-05 00:13, Alvaro Herrera wrote:
> Robert Haas wrote:
>> On Tue, Aug 4, 2015 at 12:41 AM, Alvaro Herrera
>> <alvherre@2ndquadrant.com> wrote:
>
>>> The alternative is to turn the feature on automatically if it sees that
>>> the master also has it on, i.e. the value would not be what the config
>>> file says it is.  Doing this would be a bit surprising IMO, but given
>>> the behavior above maybe it's better than the current behavior.
>>
>> I think it's totally reasonable for the standby to follow the master's
>> behavior rather than the config file.  That should be documented, but
>> otherwise, no problem.  If it were technologically possible for the
>> standby to follow the config file rather than the master in all cases,
>> that would be fine, too.  But the current behavior is somewhere in the
>> middle, and that doesn't seem like a good plan.
>
> So I discussed this with Petr.  He points out that if we make the
> standby follows the master, then the problem would be the misbehavior
> that results once the standby is promoted: at that point the standby
> would no longer "follow the master" and would start with the feature
> turned off, which could be disastrous (depending on what are you using
> the commit timestamps for).
>
> Given this, we're leaning towards the idea that the standby should not
> try to follow the master at all.  Instead, an extension that wants to
> use this stuff can check the value for itself, and raise a fatal error
> if it's not already turned on the config file.  That way, a lot of the
> strange corner cases disappear.
>

Actually, after thinking bit more about this I think the behavior of 
these two will be similar - you suddenly lose the commit timestamp info. 
The difference is that with fist option you'll lose it after restart 
while with second one you lose it immediately after promotion since 
there was never any info on the slave.

Extensions can do sanity checking in both scenarios.

The way I see it the first option has following advantages:
- it's smaller change
- it's more consistent with how wal_log_hints behaves
- fixing the config does not require server restart since the in-memory 
state was set from WAL record automatically

However the second option has also some:
- one can have slave which doesn't have overhead of the commit timestamp 
SLRU if they don't need it there
- it's theoretically easier to notice that the track_commit_timestamps 
is off in config because the the SQL interface will complain if called 
on the slave

So +0.5 from me towards following master and removing the error message

-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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