+External snapshots
+------------------
+
+You can use an external _read only_ device as an origin for a
+thinly-provisioned volume. Any read to an unprovisioned area of the
+thin device will be passed through to the origin. Writes trigger
+the allocation of new blocks as usual.
+
+One use case for this is VM hosts that want to run guests on
+thinly-provisioned volumes but have the base image on another device
+(possibly shared between many VMs).
+
+You must not write to the origin device if you use this technique!
+Of course, you may write to the thin device and take internal snapshots
+of the thin volume.
+
+i) Creating a snapshot of an external device
+
+ This is the same as creating a thin device.
+ You don't mention the origin at this stage.
+
+ dmsetup message /dev/mapper/pool 0 "create_thin 0"
+
+ii) Using a snapshot of an external device.
+
+ Append an extra parameter to the thin target specifying the origin:
+
+ dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 0 /dev/image"
+
+ N.B. All descendants (internal snapshots) of this snapshot require the
+ same extra origin parameter.
+