DASD device driver
  
  S/390's disk devices (DASDs) are managed by Linux via the DASD device
  driver. It is valid for all types of DASDs and represents them to
  Linux as block devices, namely "dd". Currently the DASD driver uses a
  single major number (254) and 4 minor numbers per volume (1 for the
  physical volume and 3 for partitions). With respect to partitions see
  below. Thus you may have up to 64 DASD devices in your system.
  
  The kernel parameter 'dasd=from-to,...' may be issued arbitrary times
  in the kernel's parameter line or not at all. The 'from' and 'to'
  parameters are to be given in hexadecimal notation without a leading
  0x.
  If you supply kernel parameters the different instances are processed
  in order of appearance and a minor number is reserved for any device
  covered by the supplied range up to 64 volumes. Additional DASDs are
  ignored. If you do not supply the 'dasd=' kernel parameter at all, the 
  DASD driver registers all supported DASDs of your system to a minor
  number in ascending order of the subchannel number.
  
  The driver currently supports ECKD-devices and there are stubs for
  support of the FBA and CKD architectures. For the FBA architecture
  only some smart data structures are missing to make the support
  complete. 
  We performed our testing on 3380 and 3390 type disks of different
  sizes, under VM and on the bare hardware (LPAR), using internal disks
  of the multiprise as well as a RAMAC virtual array. Disks exported by
  an Enterprise Storage Server (Seascape) should work fine as well.
  
  We currently implement one partition per volume, which is the whole
  volume, skipping the first blocks up to the volume label. These are
  reserved for IPL records and IBM's volume label to assure
  accessibility of the DASD from other OSs. In a later stage we will
  provide support of partitions, maybe VTOC oriented or using a kind of
  partition table in the label record.
  
  USAGE
  
  -Low-level format (?CKD only)
  For using an ECKD-DASD as a Linux harddisk you have to low-level
  format the tracks by issuing the BLKDASDFORMAT-ioctl on that
  device. This will erase any data on that volume including IBM volume
  labels, VTOCs etc. The ioctl may take a 'struct format_data *' or
  'NULL' as an argument.  
  typedef struct {
  	int start_unit;
  	int stop_unit;
  	int blksize;
  } format_data_t;
  When a NULL argument is passed to the BLKDASDFORMAT ioctl the whole
  disk is formatted to a blocksize of 1024 bytes. Otherwise start_unit
  and stop_unit are the first and last track to be formatted. If
  stop_unit is -1 it implies that the DASD is formatted from start_unit
  up to the last track. blksize can be any power of two between 512 and
  4096. We recommend no blksize lower than 1024 because the ext2fs uses
  1kB blocks anyway and you gain approx. 50% of capacity increasing your
  blksize from 512 byte to 1kB.
  
  -Make a filesystem
  Then you can mk??fs the filesystem of your choice on that volume or
  partition. For reasons of sanity you should build your filesystem on
  the partition /dev/dd?1 instead of the whole volume. You only lose 3kB	
  but may be sure that you can reuse your data after introduction of a
  real partition table.
  
  BUGS:
  - Performance sometimes is rather low because we don't fully exploit clustering
  
  TODO-List:
  - Add IBM'S Disk layout to genhd
  - Enhance driver to use more than one major number
  - Enable usage as a module
  - Support Cache fast write and DASD fast write (ECKD)