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

To: "Alasdair G Kergon" <agk@xxxxxxxxxx>
Subject: Re: [dm-devel] [PATCH 0/3] freeze feature ver 1.8
From: "Takashi Sato" <t-sato@xxxxxxxxxxxxx>
Date: Tue, 1 Jul 2008 18:12:32 +0900
Cc: <akpm@xxxxxxxxxxxxxxxxxxxx>, <viro@xxxxxxxxxxxxxxxxxx>, <axboe@xxxxxxxxx>, <mtk.manpages@xxxxxxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>, <xfs@xxxxxxxxxxx>, <dm-devel@xxxxxxxxxx>, <linux-fsdevel@xxxxxxxxxxxxxxx>, <linux-ext4@xxxxxxxxxxxxxxx>
In-reply-to: <20080630135433.GA22522@xxxxxxxxxxxxxxxxxx>
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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.


Cheers, Takashi

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