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Re: [HACKERS] Re: BUG #4796: Recovery followed by backup creates unrecoverable WAL-file

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Mikael Krantz <mk(at)zigamorph(dot)se>, pgsql-bugs(at)postgresql(dot)org, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Re: BUG #4796: Recovery followed by backup creates unrecoverable WAL-file
Date: 2009-05-07 15:57:20
Message-ID: 4A0304E0.8050301@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackers
Simon Riggs wrote:
> On Thu, 2009-05-07 at 17:54 +0300, Heikki Linnakangas wrote:
>> Simon Riggs wrote:
>>> A more useful thing might be to do an xlog switch before we do the
>>> shutdown checkpoint at end of recovery. That gives the same sequence of
>>> actions without modifying the existing sequence of activities for
>>> backups, which is delicate enough for me to not want to touch it.
>>
>> Hmm, yeah should work as well. I find the recovery sequence to be even 
>> more delicate, though, than pg_start_backup(). I think you'd need to 
>> write the XLOG switch record using the old timeline ID, as we currently 
>> require that the timeline changes only at a shutdown checkpoint record. 
>> That's not hard, but does make me a bit nervous.
>
> Yes, you're right about the delicacy of all of this so both suggestions
> sound kludgey - the problem is to do with timelines not with sequencing
> of checkpoints and log switches. The problem is Mikael deleted the
> history file and he shouldn't have done that. 

I don't see any user error here. What he did was:

1. Restore from backup A
2. Clear old WAL archive
3. pg_start_backup() + tar all but pg_xlog + pg_stop_backup();
4. Restore new backup B

There's no history file in the archive because it was cleared in step 2. 
There's nothing wrong with that; you only need to retain WAL files from 
the point that you call pg_start_backup(). There's no history file 
either in the tar, because pg_xlog was not tarred as we recommend in the 
manual.

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

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