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

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Date:	Tue, 1 Jul 2008 18:12:32 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Alasdair G Kergon" <agk@...hat.com>
Cc:	<akpm@...ux-foundation.org>, <viro@...IV.linux.org.uk>,
	<axboe@...nel.dk>, <mtk.manpages@...glemail.com>,
	<linux-kernel@...r.kernel.org>, <xfs@....sgi.com>,
	<dm-devel@...hat.com>, <linux-fsdevel@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>
Subject: Re: [dm-devel] [PATCH 0/3] freeze feature ver 1.8

Hi,

Alasdair G Kergon 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?

Yes, you are right.
We'd like to use the freeze feature without device-mapper/LVM.

> 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

Cheers, Takashi 
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