Plin idisks
Track Characteristics
Like real disks, minidisks must be formatted for use by the appropriate
service program. A minidisk is initialized for use by executing one of
the following service programs in a virtual machine: For all CPlS formats the
records.
disks except CPlS/VSAPI specified tracks into
disks, the CMS FORMAT 800-byte blocks or
command
physical For Cp disKs, t.he standalone CP Format/Allocate program must be used
to format specified tracks into 4096-byte blocks. For OS, DOS, and CMS/VSAM minidisks the IBCDASDI service program
writes read-only track descriptor records for each track, and clears
the remaining portion of each track to binary zeros. It also writes
a format-5 DSCB whose contents reflect the minidisk size (the amount
of free space available for allocation). Any disk initialization
program that supports the operatinq systems use of the DASD device
type may be used if you are initializing full disks.
Minidisks defined in the VM/370 directory are initialized only once;
temporary minidisks must be initialized each time they are used.
Alternate Tracks 3330/3350 DISKS Alternate tracks assigned at the factory or by IBCDASDI in the field are
automatically handled on the 3330 or 3350 by the control unit.
Minidisks on the 3330 .Model 1 or 2 should be specified on cylinder 0 through cylinder 403 only. The remaining cylinders (404 to 411) are
automatically used by the 3830 Control Unit for alternate tracks. Minidisks on the 3330 Model 11 can be specified on cylinder 0 through
cylinder 807. Minidisks on the 3350 should be specified on cylinder 0 through cylinder 554 only. The remaining cylinders (555 to 559) are
automatically used by the 3830 control unit for alternate tracks. 3340 DISKS The 3340 DASD device uses a hardware logic that lessens the dependence
on alternate track usage. The 3340 can bypass the defective portion of
a data track and write the balance of the record in the space remaining.
In the case where an alternate track is required, the alternate track
can be assigned by IBCDASDI standalone using a dedicated 3340 device.
Cylinder 348 on the 3348 Data Module, Model 35 and cylinders 696 and 697
on the 3348 Data Module, Model 70 are reserved for this purpose. Once IBCDADSI has assigned the alternate track, the disk, including the
cylinder containing the defective track, may be used for any purpose
whatever, including CP system residence, CMS minidisks, and so forth.
There are only two restrictions: A minidisk should not be located where its track 0, cylinder 0 falls
on a defective track because then it will be impossible for the CP IPL command to function for that minidisk. Any operating system doing 510 to this disk must be capable of doing
the normal alternate track error recovery. eMS qualifies here because it uses DIAGNOSE in place of SIO. 98 IBM VM/370 Planning and System Generation Guide
Minidisks When an attempt to do I/O on a defective 3340 or 3344 track results in a
track condition check, software error recovery procedures provide for
switching to an alternate track. For CP I/O and for diagnose I/O issued
from a virtual machine, the switching is fully automatic and the issuer o-f the I/O request is not aware of it. For SIO issued from a virtual
machine, a track condition check is reflected to the virtual machine so
that the operating system in the virtual machine will run its own error
recovery procedures. Since alternate tracks are assigned from the high-order cylinders at
the end of the real 3340, the virtual machine will attempt to seek
outside of the minidisk to recover. The VK/370 CCW translation process
allows seeks outside of the minidisk to an alternate track provided that
the particular alternate track is assigned to a defective track within
that minidisk. After seeking to the alternate track, any attempts at
head switching to an unowned track in this cylinder are prevented. On 3340-35 devices, the primary data area is cylinder 0-347. Cylinder
348 is reserved for alternate tracks. On 3340-70 devices or 3344
devices, the primary data area is cylinder 0-695. Cylinders 696-691 are
reserved for alternate tracks.
Previously, the "alternate tracks" cylinders of 3340/3344 devices were
often used as primary data cylinders, but now these cylinders must be
reserved exclusively for alternate track use. Therefore, when changing
from an old system (prior to Release 5 PLC 6) to a current system, it is
necessary to revise the space allocation and minidisk layouts on any 3340/3344 disk where the "alternate tracks" cylinders had been used as a
primary data area. If the system residence device contains Ital ternate tracks" cylinders that have been used as the primary data
area, the files of existing control statements should be revised prior
to generating a new system. In particular, the allocate function
performed on the system residence disk and other CP-owned disks may have
to be revised, and subsequent to this revision, the specification of the SYSRES, NAKESYS, and NAKENCP macros should be reviewed. If any minidisks on a 3340/3344 extend into the
alternate tracks cylinders, they can be copied to another area of the
disk or to another disk using the DASD dump restore (DDR) utility. In
the past, when a 3340/3344 had a defective track, the cylinder with the
bad track was unusable and minidisks would be allocated adjacent to that
cylinder, but not including it. In this case, all cylinders of the real
disk should be dumped to tape using any version of the DDR utility.
If you use the new version of the DDR utility and the alternate
tracks cylinders have been used as a primary data area, make sure that
you specify the cylinder range explicitly. For example, enter: DUKP 0 TO 697 Part 1. Planning for System Generation 99
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