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-rw-r--r--Documentation/device-mapper/thin-provisioning.txt39
1 files changed, 38 insertions, 1 deletions
diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt
index a119fbdd060..310c1201d62 100644
--- a/Documentation/device-mapper/thin-provisioning.txt
+++ b/Documentation/device-mapper/thin-provisioning.txt
@@ -169,6 +169,38 @@ ii) Using an internal snapshot.
dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 1"
+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.
+
Deactivation
------------
@@ -254,7 +286,7 @@ iii) Messages
i) Constructor
- thin <pool dev> <dev id>
+ thin <pool dev> <dev id> [<external origin dev>]
pool dev:
the thin-pool device, e.g. /dev/mapper/my_pool or 253:0
@@ -263,6 +295,11 @@ i) Constructor
the internal device identifier of the device to be
activated.
+ external origin dev:
+ an optional block device outside the pool to be treated as a
+ read-only snapshot origin: reads to unprovisioned areas of the
+ thin target will be mapped to this device.
+
The pool doesn't store any size against the thin devices. If you
load a thin target that is smaller than you've been using previously,
then you'll have no access to blocks mapped beyond the end. If you