[linux-lvm] Using LVM to simulate Copy-on-Write

Rik Herrin rikherrin at yahoo.com
Sun Mar 18 19:18:35 UTC 2007

  I was wondering if there was some way to use LVM to
simulate the Copy-on-Write technique used in User Mode
Linux.  Basically, the idea is as follows:

"The way to share a filesystem between two virtual
machines is to use the copy-on-write (COW) layering
capability of the ubd block driver. As of 2.4.6-2um,
the driver supports layering a read-write private
device over a read-only shared device. A machine's
writes are stored in the private device, while reads
come from either device - the private one if the
requested block is valid in it, the shared one if not.
Using this scheme, the majority of data which is
unchanged is shared between an arbitrary number of
virtual machines, each of which has a much smaller
file containing the changes that it has made. With a
large number of UMLs booting from a large root
filesystem, this leads to a huge disk space saving. It
will also help performance, since the host will be
able to cache the shared data using a much smaller
amount of memory, so UML disk requests will be served
from the host's memory rather than its disks. " [Taken

Any ideas?  Thanks for your help.

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