This is new patchset for the shared exception store.

You can find the first submission:

https://www.redhat.com/archives/dm-devel/2008-August/msg00003.html

Seems that there is a chance that the shared exception store feature
might be merged into mainline if it's implemented nicely so this time
I try to integrate the feature into the existing code more cleanly and
submit patches in a reasonable way.

There are 10 patches and #1-7 patches include the changes to the
existing code necessary to support the shared exception dm-snapshot.

My main focus is still a way to incorporate this new feature into the
exisiting code nicely.

The patches are available (against the latest git tree):

http://www.kernel.org/pub/linux/kernel/people/tomo/dm-snap/

If you like a git tree:

git://git.kernel.org/pub/scm/linux/kernel/git/tomo/linux-2.6-misc.git dm-snap


=
In this example, /dev/sda1 is used as an origin device and /dev/sdb1
is used as a cow device.

- first, set up an origin and a cow:

vine:/home/fujita# echo |dmesg create --notable work
vine:/home/fujita# echo 0 `blockdev --getsize /dev/sda1` s-snap-origin /dev/sda1 /dev/sdb1 S 16|dmsetup load work
vine:/home/fujita# dmsetup resume work


- confirm the current status:

vine:/home/fujita# dmsetup status
work: 0 268430022 s-snap-origin : no snapshot


- create one snapshot (the id is 0):

vine:/home/fujita# dmsetup message work 0 snapshot create 0


- see if the snapshot is created:

vine:/home/fujita# dmsetup status
work: 0 268430022 s-snap-origin 1 0


- create one more snapshot (the id is 5) and see the result:

vine:/home/fujita# dmsetup message work 0 snapshot create 5
vine:/home/fujita# dmsetup status
work: 0 268430022 s-snap-origin 2 0 5

Note that creating a snapshot just means that you store some
information on the cow device. You don't create a device for it yet.


- create a device to access to the snapshot #5

vine:/home/fujita# echo |dmsetup create --notable snap5
vine:/home/fujita# echo 0 `blockdev --getsize /dev/sda1` s-snap /dev/sda1 5|dmsetup load snap5
vine:/home/fujita# dmsetup resume snap5

- confirm the current status:

vine:/home/fujita# dmsetup status
snap5: 0 268430022 s-snap Unknown
work: 0 268430022 s-snap-origin 2 0 5


- remove the devices after you play with them:

vine:/home/fujita# dmsetup remove snap5
vine:/home/fujita# dmsetup remove work
vine:/home/fujita# dmsetup info
No devices found


- Again, setup the origin and the cow:

vine:/home/fujita# echo |dmsetup create --notable work
vine:/home/fujita# echo 0 `blockdev --getsize /dev/sda1` s-snap-origin /dev/sda1 /dev/sdb1 S 16|dmsetup load work
vine:/home/fujita# dmsetup resume work
vine:/home/fujita# dmsetup status
work: 0 268430022 s-snap-origin 2 0 5

Note that you have two snapshots that you created. You can play with
them again after creating devices for them.


*) By default, dmsetup perfomrs multiple tasks. I uses dmsetup
commands that issue a single ioctl to make things clear.



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