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Re: [dm-devel] RFC: UUID rename



On Fri, Feb 28, 2003 at 11:27:39AM -0600, Kevin Corry wrote:
> Hi Joe,
> 
> I have a new feature request for Device-Mapper that I'm hoping you'll
> consider. Currently, the DM ioctl interface allows device's to be renamed,
> but the UUID remains unchanged for the life of the device. This patch
> adds the ability to change the UUID as well as the name.

I'm really not keen on this at all.  One of the few things I liked
about the filesystem interface was that we'd managed to get rid of the
concept of UUIDs completely by allowing arbitrary device namespaces.
So if you use UUIDs, you created a UUID namespace and aliased the
devices however you want.

At this point I can't see the need for your change.

> Another reason for the change is due to our implementation of snapshotting.
> When taking a snapshot of a device (we'll call that device the origin child),
> we'd like the newly created snapshot-origin (we'll call it the origin parent)
> to effectively switch device numbers with the child. This way, any active
> users of the origin child will not notice that the snapshot has taken place.
> However, the device numbers cannot be changed. Thus, we create the parent
> device with the child's mapping, perform a series of renames, and reload the
> child device with a snapshot-origin map. But in order for the renames to work
> correctly, both the names and uuids must be switched.

Why can't you do the snapshotting the same way that LVM2 does it ?


   Logical volume uuid = 1    -->     Origin volume uuid = 1
                                                |
						\/
					Logical volume

so you:

i) suspend the lv (uuid==1)
ii) create a new device with the lv table
iii) reload an origin mapping over the original lv (uuid==1)
iv) resume (uuid==1)

- Joe



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