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diff --git a/Documentation/s390/dasd.rst b/Documentation/s390/dasd.rst new file mode 100644 index 000000000000..9e22247285c8 --- /dev/null +++ b/Documentation/s390/dasd.rst @@ -0,0 +1,84 @@ +================== +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) |