linux-ext4 - Re: [dm-devel] [PATCH 0/3] freeze feature ver 1.8

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Date:	Mon, 30 Jun 2008 14:54:33 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"mtk.manpages@...glemail.com" <mtk.manpages@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [dm-devel] [PATCH 0/3] freeze feature ver 1.8

On Mon, Jun 30, 2008 at 09:20:05PM +0900, Takashi Sato wrote:
> 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.
> In many case, a commercial filesystem (e.g. VxFS) has
> the freeze feature and it would be used to get the consistent backup.
> If Linux's standard filesytem ext3 has the freeze feature, we can do it
> without a commercial filesystem.
 
Is the following a fair summary?

1. Some filesystems have a freeze/thaw feature.  XFS exports this to
userspace directly through a couple of ioctls, but other filesystems
don't.  For filesystems on device-mapper block devices it is exported to
userspace through the DM_DEV_SUSPEND ioctl which LVM uses.

2. There is a desire to access this feature from userspace on non-XFS
filesystems without having to use device-mapper/LVM.

Alasdair
-- 
agk@...hat.com
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