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