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Re: Streaming replication, and walsender during recovery

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication, and walsender during recovery
Date: 2010-01-29 07:49:32
Message-ID: 4B62930C.40302@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs wrote:
> On Thu, 2010-01-28 at 21:00 +0200, Heikki Linnakangas wrote:
>> I think it is a pretty important safety feature that we keep all the
>> WAL around that's needed to recover the standby. To avoid
>> out-of-disk-space situation, it's probably enough in practice to set
>> checkpoint_timeout small enough in the standby to trigger
>> restartpoints often enough.
> 
> Hmm, I'm sorry but that's bogus. Retaining so much WAL that we are
> strongly in danger of blowing disk space is not what I would call a
> safety feature. Since there is no way to control or restrain the number
> of files for certain, that approach seems fatally flawed.

The other alternative is to refuse to recover if the master can't be
contacted to stream the missing WAL again. Surely that's worse.

Note that we don't have any hard limits on WAL disk usage in general.
For example, if archiving stops working for some reason, you'll
accumulate WAL in the master until it runs out of disk space.

> Reducing
> checkpoint_timeout is the opposite of what you would want to do for
> performance.

Well, make sure you have enough disk space for a higher setting then. It
doesn't seem that hard.

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

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