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Re: BUG #6170: hot standby wedging on full-WAL disk

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Daniel Farina <daniel(at)heroku(dot)com>, pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #6170: hot standby wedging on full-WAL disk
Date: 2011-08-25 17:16:45
Message-ID: 4E56837D.5010706@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-bugs
On 25.08.2011 19:11, Robert Haas wrote:
> On Mon, Aug 22, 2011 at 2:57 AM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com>  wrote:
>> So the problem is that walreceiver merrily writes so much future WAL that it
>> runs out of disk space? A limit on the maximum number of future WAL files to
>> stream ahead would fix that, but I can't get very excited about it. Usually
>> you do want to stream as much ahead as you can, to ensure that the WAL is
>> safely on disk on the standby, in case the master dies. So the limit would
>> need to be configurable.
>
> It seems like perhaps what we really need is a way to make replaying
> WAL (and getting rid of now-unneeded segments) to take priority over
> getting new ones.

With the defaults we start to kill queries after a while that get in the 
way of WAL replay. Daniel had specifically disabled that. Of course, 
even with the query-killer disabled, it's possible for the WAL replay to 
fall so badly behind that you fill the disk, so a backstop might be 
useful anyway, although that seems a lot less likely in practice and if 
your standby can't keep up you're in trouble anyway.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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