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[Xen-devel] testing that volumes are not currently attached to loopback

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Subject: [Xen-devel] testing that volumes are not currently attached to loopback devices
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Thu, 9 Apr 2009 22:42:58 +1000
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Thread-index: Acm5ELYCpXoIbRcqRsSNkDYTuxdj1A==
Thread-topic: testing that volumes are not currently attached to loopback devices
xend will not start a domain if one of the block devices is currently
mounted elsewhere, but it will not detect if the device is currently
attached to a loopback device. I wouldn't have thought this a problem,
provided that the loop device itself wasn't in use, but it causes all
sorts of strange things to happen.

My loop device was mounted with the following command 'losetup --offset
32256 /dev/loop4 /dev/vg00/virt-domu_name-0', which allows me to mount
the filesystem with ntfs-3g (32256 is the offset of the partition in the
disk image).

The symptoms were that windows would BSoD shortly after login, chkdsk
always reported errors (as done under the recovery console) and changes
made in the DomU filesystem didn't seem to stick. As soon as I
un-attached the loopback device (losetup -d) all those problems went
away. For some strange reason this didn't result in the total
destruction of the filesystem :)

The box I'm testing this on is 3.1.x... is it fixed in any later
versions? Any idea how to detect if a volume is attached via losetup?
Any idea why this causes a problem at all?



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