+Select volumes from which to collect snapshot information.
+
+Usually, the space used for snapshots is included in the space reported as
+"used". If snapshot information is collected as well, the space used for
+snapshots is subtracted from the used space.
+
+To make things even more interesting, it is possible to reserve space to be
+used for snapshots. If the space required for snapshots is less than that
+reserved space, there is "reserved free" and "reserved used" space in addition
+to "free" and "used". If the space required for snapshots exceeds the reserved
+space, that part allocated in the normal space is subtracted from the "used"
+space again.