Setting a buffer_heads 'real_device'

Hi,

I'm writing a virtual disk driver consisting of serveral real disks.
Hence I've to map read/write requests for my virtual device to
underlying
real devices, like md does so for RAID-Arrays.

So my question is how a virtual device driver (like 'md') manages the
read/write
request associated with a virtual (md-) device to get data from a
specific real
device within a 'block_read/block/write()' invokation !
I know that the buffer head-structure contains an entry for a virtual
and a real
device, but i don't know when and in which proper way to set these
components.

Somebody to help me ??

Mario Vodisek