Take a snapshot of an existing **volume**. This operation will fail if the
back-end to which the **volume** belongs to does not support snapshotting.
Note that you cannot specify both *NAME* and *DEST* since those options are
mutually exclusive.

In addition to specifying new size of the volume directly, percentage can be
specified as well. Specify **--size 70%** to indicate the new snapshot size
to be 70% of the origin volume size. Additionally, percentage of the used,
or free pool space can be specified as well using keywords FREE, or USED
respectively.

In some cases the file system has to be mounted in order to take a snapshot of
the **volume**. This will be handled by **ssm** automatically by mounting the
**volume** temporarily.
