Subject: Re: Restricting maximum keep segments by repslots
To: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
 pgsql-hackers@postgresql.org
References: <20170228.122736.123383594.horiguchi.kyotaro@lab.ntt.co.jp>
From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Message-ID: <33bcf803-4aa8-27b6-b6bd-4fb3ae950b19@2ndquadrant.com>
Date: Tue, 28 Feb 2017 05:27:05 +0100
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On 28/02/17 04:27, Kyotaro HORIGUCHI wrote:
> Hello.
> 
> Although replication slot is helpful to avoid unwanted WAL
> deletion, on the other hand it can cause a disastrous situation
> by keeping WAL segments without a limit. Removing the causal
> repslot will save this situation but it is not doable if the
> standby is active. We should do a rather complex and forcible
> steps to relieve the situation especially in an automatic
> manner. (As for me, specifically in an HA cluster.)
> 

I agree that that it should be possible to limit how much WAL slot keeps.

> This patch adds a GUC to put a limit to the number of segments
> that replication slots can keep. Hitting the limit during
> checkpoint shows a warining and the segments older than the limit
> are removed.
> 
>> WARNING:  restart LSN of replication slots is ignored by checkpoint
>> DETAIL:  Some replication slots lose required WAL segnents to continue.
> 

However this is dangerous as logical replication slot does not consider
it error when too old LSN is requested so we'd continue replication,
hiding data loss.

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


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