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I use dmsetup's snapshot target to try out changes to a disk images before committing them (the primary use case is when doing data recovery - repairing the image filesystem without risking any data).

The image is loop'd read-only, (and I also loop a backing file for the snapshot backing file) and create a snapshot like so:

$ echo 0 <size> snapshot <image-loop-device> <backing-loop-device> P 1 | \
 dmsetup create <name>

It works fine: changes to <name> are saved in <backing-loop-device>, and <image-loop-device> is untouched.

However, if the <image-loop-device> is mounted, then dmsetup fails, with a Device or resource busy error. Why should dmestup should need an exclusive lock on the image device and is there a way around it (other than using a separate loop device for the mount)?

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