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

To: Takashi Sato <t-sato@xxxxxxxxxxxxx>
Subject: Re: [dm-devel] [PATCH 0/3] freeze feature ver 1.8
From: Alasdair G Kergon <agk@xxxxxxxxxx>
Date: Mon, 30 Jun 2008 14:54:33 +0100
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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.


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