Documentation/s390: remove outdated dasd documentation

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DASD device driver

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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)

    vfio-ap

    vfio-ccw

    zfcpdump

    dasd

    common_io

    text_files