Hi,

Attached is a script from Heinz Mauelshagen to set up a sparse device
for testing purposes, allowing a given amount of storage to act as a
test bed for a much larger filesystem --- as long as you don't actually
write to all that data. :)   It basically creates a snapshot of the
empty zero device, and fills it in using the backing store you give it
for snapshot storage.

It uses the device-mapper snapshot mechanism, so with the default
snapshot chunksize of 64k it allocates 64k at a time from the backing
device to the sparse device.

Use it as:

	sparse_create $mapname $dev $blocks

which will create a new device map of $blocks 512-byte blocks,
accessible as the device /dev/mapper/$mapname and using the device $dev
as backing store.  You can hard-link the script to sparse_remove and use
it to remove the map, too.  Device mapper lets you layer logical devices
so it is quite OK to use an LVM volume for $dev.

Note that each time you write to a new 64k chunk of the sparse device,
it will cause the full 64k to get written to the backing store.
Subsequent writes only update the data actually being written, so
there's a performance hit for the first write to such a chunk.

Cheers,
 Stephen