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

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Date:	Wed, 9 Jul 2008 17:13:46 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Miklos Szeredi <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 Tue, Jul 08, 2008 at 11:55:02PM -0700, Arjan van de Ven wrote:
> 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.)

Bloody hell! Doesn't *anyone* understand that a frozen filesystem is
*clean*? That the process of freezing it ensures all dirty data and
metadata is written out before the freeze completes? And that once
frozen, it can't be dirtied until unfrozen?

That's 3 (or is it 4 - maybe 5 now that I think about it) different
ppl in 24 hours that have made this same broken argument about
being unable to write back dirty data on a frozen filesystem......

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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