Specifications

18 IBM System z Personal Development Tool: Volume 3 Additional Topics
򐂰 The method of modifying Linux boot functions differs with different Linux distributions. For
example, we might change /etc/rc.local, or /etc/rc.d/rc.local, or /etc/init.d/boot.local, or
some other file, depending on the exact Linux distribution.
򐂰 We can easily make a general change for stN and nstN devices, but we cannot easily
make a general change for sgN devices. We should not change the permissions to allow
universal access to all /dev/sgN devices; this would allow easy destruction of our Linux
system.
We could place the following lines in /etc/init.d/boot.local (assuming our particular Linux
uses this file):
chmod 666 /dev/st[0-9] change all tape devices
chmod 666 /dev/nst[0-9] change all tape devices
chmod 666 /dev/sg7 change particular SCSI device
Do not use chmod 666 /dev/sg[0-9] because this would allow anyone to directly access all
the SCSI devices in your system. Unfortunately, the sgN numbers are dynamically assigned
during Linux boot processing and depend on what devices are found (powered up) during
boot. Unless you always have exactly the same devices powered up when you boot Linux,
you cannot safely predict the sgN number of a given device.
Block counts
There has been a little confusion about zPDT usage of SCSI-attached 3592 tape drives.The
current zPDT SCSI device manager regards SCSI-attached tape drives as IBM 3490 units.
This means the actual device controls and sense information are transformed to appear as a
3490.
IBM 3490 tape drives provide a block counter. This counter is 22 bits; the largest count it can
hold is approxmately 4 million. If you write to a SCSI-attached tape drive (under zPDT) you
will receive an end-of-media indication after writing approximately 4 million blocks. At this
point z/OS will perform EOV functions and call for a new tape cartridge (depending on your
JCL, of course). The remaining tape media in the first cartridge is not used, but the system
works correctly otherwise.
If you read a 3490 cartridge written by another system (that was not emulating a 3490 drive)
the cartridge may contain more than 4 million blocks. This should work correctly provided the
zPDT System z program does not read the block count from the tape. If it does, and if the
tape position is past the 4 million block point, the System z program will indicate a block count
error. What happens at that point depends on the design of the particular application
program. A tape cartridge with more than approximately 4 million blocks is not fully
compatible with 3490 emulation.
2.2 Drives and interfaces
There is a wide range of possibilities for SCSI drives and interfaces.
The only SCSI tape drives used during development were the Fujitsu 3490E drive, the IBM
3580 LTO drives, and the IBM TS1120 drives. Although such usage is untested by IBM, other
SCSI tape drives, such as DLT units,
may operate correctly.
A key consideration for traditional (“parallel” interface) SCSI tape drives is the hardware
interface. These interfaces include HVD (high voltage differential), LVD (low voltage
differential), and single-ended (narrow or wide). The tape drive interface must match the SCSI
adapter interface in your PC, and the underlying Linux must work correctly with the tape drive.