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Re: Allowing multiple concurrent base backups

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Magnus Hagander <magnus(at)hagander(dot)net>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Allowing multiple concurrent base backups
Date: 2011-01-24 21:02:08
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Lists: pgsql-hackers
On 24.01.2011 22:31, Magnus Hagander wrote:
> On Mon, Jan 24, 2011 at 21:14, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com>  wrote:
>> On 24.01.2011 20:22, Magnus Hagander wrote:
>>> I can't see an explicit check for the user ttempting to do
>>> pg_stop_backup() when there is a nonexclusive backup running? Maybe
>>> I'm reading it wrong? The case being when a user has started a backup
>>> with pg_basebackup but then connects and manually does a
>>> pg_stop_backup. ISTM it drops us ina codepath that just doesn't do the
>>> decrement, but also doesn't throw an error?
>> It throws an error later when it won't find backup_label. For better or
>> worse, it's always been like that.
> Isn't that going to leave us in a broken state though? As in a
> mistaken pg_stop_backup() will decrement the counter both breaking the
> streaming base backup, and also possibly throwing an assert when that
> one eventually wants to do it's do_pg_stop_backup()?

Umm, no. pg_stop_backup() won't decrement the counter unless there's an 
exclusive backup in operation according to the exclusiveBackup flag.

Hmm, perhaps the code would be more readable if instead of the 
forcePageWrites counter that counts exclusive and non-exclusive backups, 
and an exclusiveBackup boolean indicating if one of the in-progress 
backups is an exclusive one, we had a counter that only counts 
non-exclusive backups, plus a boolean indicating if an exclusive backup 
is in progress in addition to them.

Attached is a patch for that (against master branch, including only xlog.c).

   Heikki Linnakangas

Attachment: more-readable-forcePageWrites.patch
Description: text/x-diff (20.1 KB)

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