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

Minidisks
rather than specifying ALL, which no longer dumps anything from the
final cylinders except tracks that have been assigned as alternates.
Then you can execute the IBCDASDI utility to assign alternate tracks to
the defective tracks so that all cylinders become usable. Subsequently,
the new DDR utility can be used to restore minidisks from the tape,
possibly reordering them into the previously unusable cylinders.
Note: Whenever a minidisk is moved to a new location or its size is changed, the corresponding MDISK statements in the system directory must
be revised. Only the new versions of the DDR, DIR, and FMT utilities should be
used with 3340/3344 devices after alternate tracks have been assigned. In release 6 the starter system has been changed
to reserve cylinder 348 for alternate track use. Therefore, the 3340 starter system can be restored to a disk that has defective tracks
(provided that alternate tracks have already been assigned by IBCDASDI) • 2314/2319 DISKS On 2314 and 2319 devices, CP and CMS (except CMS/VSAM) do not recognize
or support alternate track techniques for their own use. DOS, OS, and CMS/VSAM minidisks, however, do recognize and support alternate tracks
on these types of DASD. The IBCDASDI service program automatically
assigns the last cylinder in any minidisk as an alternate track
cylinder. When you initialize 2314/2319 devices, you can assign all 203 cylinders for virtual machine and system use.
If a track assigned to a virtual machine minidisk area subsequently
becomes defective, you can: • Run the standalone CP Format/Allocate service program if the minidisk
is used by CP, and flag the whole cylinder containing the defective
track as permanently assigned (PERM). This prevents CP from ever
allocating that cylinder for CP paging, spooling, or temporary files. You must remember not to include this cylinder when you allocate disk
space for any virtual machine's minidisk in the VM/370 directory. • If the minidisk is used by either DOS, OS, or CMS/VSAM, reformat the
minidisk (including the defective track) with the IBCDASDI service
program. An alternate track is assigned at the end of the minidisk. • Set up the entire volume containing the defective track as an OS, DOS, or CMS/VSAM volume and format it with either IBCDASDI or IEHDASDR for OS or CMS/VSAM disks, or with the DOS Initialize Disk
utility program (INTDK) for DOS disks. Alternate tracks are assigned
in the standard manner. Labels All disks to be handled by CP (as an entity or as a combination of
logical disks) must have a label on real cylinder 0, track 0, record 3.
This label identifies the physical volume to VM/370 and must be in the
form VOLlxxxxxx -- or -- CMS=xxxxxx where xxxxxx is a 6-character volume label. 100 IBM VM/370 Planning and System Generation Guide