linux-ext4 - Re: [PATCH 3/3] Add timeout feature

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Date:	Tue, 8 Jul 2008 23:55:02 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	miklos@...redi.hu, hch@...radead.org, tytso@....edu, pavel@...e.cz,
	t-sato@...jp.nec.com, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, mtk.manpages@...glemail.com
Subject: Re: [PATCH 3/3] Add timeout feature

On Wed, 09 Jul 2008 08:48:42 +0200
Miklos Szeredi <miklos@...redi.hu> wrote:

> On Tue, 8 Jul 2008, Arjan van de Ven wrote:
> > nah he needs to do
> > 
> > make_snapshot ; backup-command ; unref_snapshot.
> > 
> > freezing isn't the right solution for the backup problem ;)
> 
> Confused, what's freezing _is_ for then?  Patch description says:
> 
>   Currently, ext3 in mainline Linux doesn't have the freeze feature
> which suspends write requests.  So, we cannot take a backup which
> keeps the filesystem's consistency with the storage device's features
>   (snapshot and replication) while it is mounted.

I tihnk the idea there is

freeze . do the snapshot op . unfreeze . make backup of snapshot

one can argue about the need of doing the first 3 steps via a userland
loop; it sure sounds like one needs to be really careful to not do any
writes to the fs from the app that does snapshots (and that includes
doing any syscalls in the kernel that allocate memory.. just because
that already could cause unrelated data to be written from inside the
app. Not fun.)


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