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Re: Streaming replication status

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication status
Date: 2010-01-10 16:40:37
Message-ID: 4B4A0305.4040905@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs wrote:
> On Fri, 2010-01-08 at 14:20 -0800, Josh Berkus wrote:
>> On 1/8/10 1:16 PM, Heikki Linnakangas wrote:
>>> * A standby that connects to master, initiates streaming, and then sits
>>> idle without stalls recycling of old WAL files in the master. That will
>>> eventually lead to a full disk in master. Do we need some kind of a
>>> emergency valve on that?
>> WARNING: I haven't thought about how this would work together with HS yes.
> 
> I've been reviewing things as we go along, so I'm not that tense
> overall. Having said that I don't understand why the problem above would
> occur and the sentence seems to be missing a verb between "without" and
> "stalls". More explanation please.

Yeah, that sentence was broken.

> What could happen is that the standby could slowly lag behind master. 

Right, that's what I'm worried about. In the worst case it the
walreceiver process in the standby might stall completely for some
reason, e.g hardware problem or SIGSTOP by an administrator.

> We
> don't have any way of monitoring that, as yet. Setting ps display is not
> enough here.

Yeah, monitoring would be nice too. But what I was wondering is whether
we need some way of stopping that from filling the disk in master.
(Fujii-san's suggestion of a GUC to set the max. amount of WAL to keep
in the master for standbys feels good to me).

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

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