Subject: Re: BUG #13770: Extending recovery_min_apply_delay on Standby
 causes it to be unavailable for a while
To: Michael Paquier <michael.paquier@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
References: <20151111155006.2644.84564@wrigleys.postgresql.org>
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 <20151230140218.5jqkq3rjoejlrgq7@alap3.anarazel.de>
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Cc: Andres Freund <andres@anarazel.de>, Greg Clough <greg@gclough.com>,
 PostgreSQL mailing lists <pgsql-bugs@postgresql.org>,
 Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>
From: Julien Rouhaud <julien.rouhaud@dalibo.com>
Message-ID: <56D76F61.5060504@dalibo.com>
Date: Wed, 2 Mar 2016 23:55:29 +0100
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On 02/01/2016 13:14, Michael Paquier wrote:
> On Thu, Dec 31, 2015 at 8:13 AM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> On Thu, Dec 31, 2015 at 12:35 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Andres Freund <andres@anarazel.de> writes:
>>>> On 2015-12-26 22:45:57 +0900, Michael Paquier wrote:
>>>>> Depending on the use cases, it may be interesting to have a switch
>>>>> allowing to not apply the delay should a consistent point not be
>>>>> reached though...
>>>
>>>> Is there actually any case where it's interesting to delay in that
>>>> scenario? I mean that really can only happen if you changed the
>>>> configuration to a different delay, or your clock offset
>>>> changed. Otherwise we should always reach the consistent point before
>>>> the delay plays a role.  I'm tempted to simply only check for delay when
>>>> consistent.
>>>
>>> The argument for having a delay at all is to allow backing up to some
>>> earlier point in the master's history; but a slave that is not yet
>>> consistent cannot provide any rollback/recovery option.  The slave is
>>> completely useless for any purpose until it reaches consistency, so
>>> it might as well do that as fast as possible, and then sit on the
>>> next WAL record until the delay is met.  +1 for no delay at all when
>>> not consistent.
>>
>> OK, I don't mind doing so if you guys think that's more adapted. Based
>> on reading the code, it seems obvious though that this was made so as
>> a delay is taken into account even before the node is consistent.
> 
> Changing my mind after more thoughts on the matter, it seems indeed
> that it would make more sense to apply delays only once the database
> has reached a consistent state to be able to do immediately
> transaction-related operations on a standby without having to wait for
> it to reach consistency for perhaps a couple of hours. Please see
> attached a patch to do that.
> 
> 

I just reviewed the patch. It's pretty straightforward and works as
intended, so I mark it as ready for committer.


-- 
Julien Rouhaud
http://dalibo.com - http://dalibo.org


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