Changelog since version 1.68 mark II:
  - merge Mike Anderson's probe() cleanup
     - num_devs is now "per host"
     - num_devs is sysfs writeable
  - add slave_alloc skeleton code

So to simulate 154 disks (for example) one might use:
  # modprobe scsi_debug add_host=11 num_devs=14

With max_luns at its default value of 2, 14 is the
maximum number of devices per host scsi_debug will
respond to (i.e. 7 targets, each with 2 lus).

Documentation updated at:
http://www.torque.net/sg/sdebug25.html