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

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, 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: 2009-05-15 14:17:36
Message-ID: 4A0D7980.4000900@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackers

Simon Riggs wrote:
> On Fri, 2009-05-15 at 22:56 +0900, Fujii Masao wrote:
>
>   
>> OK, I probably understood your point. The timeline history files whose
>> timeline ID is larger than that of an oldest backup must not be deleted
>> from the archive. On the other hand, the smaller or equal one can be
>> deleted. Not all history files are necessary. So, if we don't keep older
>> backup, we probably can delete all files in the archive before
>> pg_start_backup().
>> Is my understanding right?
>>     
>
> Heikki is right in one sense: if you do pg_start_backup() then for
> *that* backup you do not need earlier files. 
>
> However, as you have pointed out, if you have *multiple* backups then
> deleting history files may cause problems with an earlier backup.
>
> It's standard practice to have >1 backup, so there is potential for
> error and minimum is we must document that. 
>
> Rather than explaining the problem and the rules by which we can work
> out exactly which history files to keep, I think it is safer to say that
> we must keep all history files.
>
>   

This whole area is unfortunately way too fragile. We need some way of 
managing these facilities that hides a lot of these details and is 
therefore less likely to produce shot feet, IMNSHO. I get very nervous 
every time I have to touch it.

cheers

andrew

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