linux-kernel - Re: how do versioning filesystems take snapshot of opened files?

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Message-ID: <18058.51079.350424.273688@notabene.brown>
Date:	Wed, 4 Jul 2007 08:02:47 +1000
From:	Neil Brown <neilb@...e.de>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	"Xin Zhao" <uszhaoxin@...il.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: how do versioning filesystems take snapshot of opened files?

On Tuesday July 3, chris.mason@...cle.com wrote:
> 
> Getting a snapshot that is useful with respect to application data
> requires help from the application.

Certainly.

>                                      The app needs to be shutdown or
> paused prior to the snapshot and then started up again after the
> snapshot is taken.

Alternately, the app needs to be able to cope with unexpected system
shutdown (aka crash) and the same ability will allow it to cope with
an atomic snapshot.  It may be able to recover more efficiently from
an expected shutdown, so being able to tell the app about an impending
snapshot is probably a good idea, but it should be advisory only.

NeilBrown
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