instances: the memory holding a struct disk should be pre-zeroed so
as to initialize any storage framework private data in the structures
properly. In addition, the memory must be writable so that the
private data may be updated.
Pointed out by: phk
APIs permit disk device drivers to register and deregister storage devices
for use by storage device consumers. No doubt this API will change
more as time flies by, but this should be helpful to the creators of
new storage device drivers.
Reviewed by: phk