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dm thin: support read only external snapshot origins

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Support the use of an external _read only_ device as an origin for a thin
device.

Any read to an unprovisioned area of the thin device will be passed
through to the origin.  Writes trigger allocation of new blocks as
usual.

One possible use case for this would be VM hosts that want to run
guests on thinly-provisioned volumes but have the base image on another
device (possibly shared between many VMs).

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
This commit is contained in:
Joe Thornber
2012-03-28 18:41:28 +01:00
committed by Alasdair G Kergon
parent c4a69ecdb4
commit 2dd9c257fb
2 changed files with 109 additions and 15 deletions
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39
Documentation/device-mapper/thin-provisioning.txt
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@@ -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