1 DASD device driver
2
3 S/390's disk devices (DASDs) are managed by Linux via the DASD device
4 driver. It is valid for all types of DASDs and represents them to
5 Linux as block devices, namely "dd". Currently the DASD driver uses a
6 single major number (254) and 4 minor numbers per volume (1 for the
7 physical volume and 3 for partitions). With respect to partitions see
8 below. Thus you may have up to 64 DASD devices in your system.
9
10 The kernel parameter 'dasd=from-to,...' may be issued arbitrary times
11 in the kernel's parameter line or not at all. The 'from' and 'to'
12 parameters are to be given in hexadecimal notation without a leading
13 0x.
14 If you supply kernel parameters the different instances are processed
15 in order of appearance and a minor number is reserved for any device
16 covered by the supplied range up to 64 volumes. Additional DASDs are
17 ignored. If you do not supply the 'dasd=' kernel parameter at all, the
18 DASD driver registers all supported DASDs of your system to a minor
19 number in ascending order of the subchannel number.
20
21 The driver currently supports ECKD-devices and there are stubs for
22 support of the FBA and CKD architectures. For the FBA architecture
23 only some smart data structures are missing to make the support
24 complete.
25 We performed our testing on 3380 and 3390 type disks of different
26 sizes, under VM and on the bare hardware (LPAR), using internal disks
27 of the multiprise as well as a RAMAC virtual array. Disks exported by
28 an Enterprise Storage Server (Seascape) should work fine as well.
29
30 We currently implement one partition per volume, which is the whole
31 volume, skipping the first blocks up to the volume label. These are
32 reserved for IPL records and IBM's volume label to assure
33 accessibility of the DASD from other OSs. In a later stage we will
34 provide support of partitions, maybe VTOC oriented or using a kind of
35 partition table in the label record.
36
37 USAGE
38
39 -Low-level format (?CKD only)
40 For using an ECKD-DASD as a Linux harddisk you have to low-level
41 format the tracks by issuing the BLKDASDFORMAT-ioctl on that
42 device. This will erase any data on that volume including IBM volume
43 labels, VTOCs etc. The ioctl may take a 'struct format_data *' or
44 'NULL' as an argument.
45 typedef struct {
46 int start_unit;
47 int stop_unit;
48 int blksize;
49 } format_data_t;
50 When a NULL argument is passed to the BLKDASDFORMAT ioctl the whole
51 disk is formatted to a blocksize of 1024 bytes. Otherwise start_unit
52 and stop_unit are the first and last track to be formatted. If
53 stop_unit is -1 it implies that the DASD is formatted from start_unit
54 up to the last track. blksize can be any power of two between 512 and
55 4096. We recommend no blksize lower than 1024 because the ext2fs uses
56 1kB blocks anyway and you gain approx. 50% of capacity increasing your
57 blksize from 512 byte to 1kB.
58
59 -Make a filesystem
60 Then you can mk??fs the filesystem of your choice on that volume or
61 partition. For reasons of sanity you should build your filesystem on
62 the partition /dev/dd?1 instead of the whole volume. You only lose 3kB
63 but may be sure that you can reuse your data after introduction of a
64 real partition table.
65
66 BUGS:
67 - Performance sometimes is rather low because we don't fully exploit clustering
68
69 TODO-List:
70 - Add IBM'S Disk layout to genhd
71 - Enhance driver to use more than one major number
72 - Enable usage as a module
73 - Support Cache fast write and DASD fast write (ECKD)
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