Subject: Re: max_worker_processes on the standby

From: Fujii Masao <[email-masked]>
Date: Thu, 3 Sep 2015 22:03:44 +0900

On Sat, Aug 8, 2015 at 11:02 PM, Robert Haas <[email-masked]> wrote:
> On Tue, Aug 4, 2015 at 6:13 PM, Alvaro Herrera <[email-masked]> wrote:
>>> 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).
>
> That seems like an imaginary problem.  If it's critical to have commit
> timestamps, don't turn them off on the standby.
>
>> To solve that problem, you could suggest that if we see the setting
>> turned on in pg_control then we should follow that instead of the config
>> file; but then the problem is that there's no way to turn the feature
>> off.  And things are real crazy by then.
>
> There's no existing precedent for a feature that lets the standby be
> different from the master *in any way*.  So I don't see why we should
> start here.  I think the reasonable definition is that the GUC
> controls whether the master tries to update the SLRU (and generate
> appropriate WAL records, presumably).  The standby should not get a
> choice about whether to replay those WAL records.

+1

I added this to the 9.5 open item list.

Regards,

-- 
Fujii Masao

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---

From: Petr Jelinek <[email-masked]>
Date: Wed, 16 Sep 2015 05:12:34 +0200

> I added this to the 9.5 open item list.

I see I forgot to send patch for this, so here it is. It just removes 
the on start check for track_commit_timestamp being same in config and 
control file.

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

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