Introduction
============
-This document descibes a collection of device-mapper targets that
+This document describes a collection of device-mapper targets that
between them implement thin-provisioning and snapshots.
The main highlight of this implementation, compared to the previous
As a guide, we suggest you calculate the number of bytes to use in the
metadata device as 48 * $data_dev_size / $data_block_size but round it up
-to 2MB if the answer is smaller. The largest size supported is 16GB.
+to 2MB if the answer is smaller. If you're creating large numbers of
+snapshots which are recording large amounts of change, you may find you
+need to increase this.
-If you're creating large numbers of snapshots which are recording large
-amounts of change, you may need find you need to increase this.
+The largest size supported is 16GB: If the device is larger,
+a warning will be issued and the excess space will not be used.
Reloading a pool table
----------------------
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
------------
Deletes a thin device. Irreversible.
- trim <dev id> <new size in sectors>
-
- Delete mappings from the end of a thin device. Irreversible.
- You might want to use this if you're reducing the size of
- your thinly-provisioned device. In many cases, due to the
- sharing of blocks between devices, it is not possible to
- determine in advance how much space 'trim' will release. (In
- future a userspace tool might be able to perform this
- calculation.)
-
set_transaction_id <current id> <new id>
Userland volume managers, such as LVM, need a way to
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
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
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