Minidisks Note! On a 3330, 3340, or 3350, an OS/VS, or OS minidisk must start at real cylinder 0 unless the VTOC is limited to one track. See the list
of restrictions "Appendix F: VM/370 Restrictions" for more
information and explanation of 3330/3340/3350 restrictions.
Minidisk Space Allocation OS bases all of its space allocation parameters on the volume table of
contents (VTOC) label written on each disk; it determines the amount of
space available on that volume from the format-5 (space accounting) data
set control block (DSCB). Thus, for OS to support minidisks, a VTOC must be written whose format-5 DSCB reflects the desired size of the
minidisk. The remainder of the disk space on the real disk appears to OS to be permanently dedicated, and not assignable by the OS space
accounting routines. The IBCDASDI service program should be used to
format minidisks for use by OS or DOS. A DASD volume containing multiple minidisks contains some tracks in
which the cylinder address in the count fields of records RO and R1 do
not agree with each other. If an attempt is made to read this volume by
IEHDASDR, you may get messages IEH8l3I or IEH869I. To prevent this,
initialize the disk with the FORMAT function of IEHDASDR before using
it. This function rewrites RO and R1 on each track so that the count
fields agree with each other. DOS space allocation is specified in the EXTENT job control card. It
is your responsibility to see that the EXTENT cards refer to valid
minidisk cylinders. On a 2314 or 2319 volume, the last cylinder of any
minidisk initialized by IBCDASDI is always reserved for use as an
alternate track cylinder. Therefore, a DOS minidisk on a 2314 or 2319,
must have a minimum of two cylinders. For example, if you are
specifying a ten-cylinder minidisk, the EXTENT card must refer to
cylinders 0 through 8 only. This leaves the last cylinder for alternate
track assignment. However, on a 3330, 3333, 3340, or 3350 minidisk,
IBCDASDI does not reserve a cylinder for alternate tracks within each
minidisk. Therefore, a ten-cylinder minidisk must be defined in the
EXTENT card as cylinders 0 through 9. A minidisk always begins at virtual cylinder zero. Its minimum size
is one cylinder unless it is located on a 2314 or 2319 disk and is
formatted by the IBCDASDI service program; in which case, the minimum
number of cylinders is two and the second cylinder is used as the
alternate track cylinder. Except for the 3350, which can be used in 3330-1 or 3330-11 compatibility mode or in native mode, a minidisk must
exist on its real counterpart, that is, a virtual 3340 minidisk must
reside on a real 3340. When minidisks are defined on MSS 3330V volumes, the minidisks are
virtual 3330-1 disks. The presence of the MSS and 3330V system volumes
is transparent to a virtual machine accessing minidisks. VM/370 controls the boundaries of minidisks. If an attempt is made
to refer to a DASD address outside the boundaries specified in the MDISK directory statements, CP presents a command reject (seek check) IIO error to the virtual machine. If the cylinder addresses in the MDISK statements inadvertently
overlap each other, the integrity of data in the overlapped cylinders
may be compromised with no error indicated. Part 1. Planning for System Generation 97
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
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