Minidisks Figure 16 illustrates the use and definition of minidisks. The disk
labeled OSDOS1 contains several minidisks, some formatted to OS requirements and others to DOS requirements. OSDOS1 is a 2314 volume. The directory entry for userid ABC (an OS user) describes the virtual
device 230 as a 50-cylinder area, and the virtual device 231 as a
20-cylinder area on real volume OSDOS1. The entry for userid XYZ (a DOS user) describes the virtual device 130 as a 50-cylinder disk
area on a real volume OSDOS1. Real
Cylinder
Number 00 Virtual
Cylinder
Number
Real
Cylinder
Number 000 001 VOL1CPVOL 1
Virtual
Cylinder
Number t 001 t DOSLIB 49 50 t
99 100 I 119 120 202 OSDOS1 49 00 DOSRES 49 00 MFTWRK
19 020 030 031 060 061 : 202 UNASSIGNED SYS1.LlNKLIB SYS1.COBLIB CP SPOOLING AREA
19 J 00 MFTSUB
29
VM/370 User Directory Entry for user ABC (An OS user) VM/370 User Directory Entry for user XYZ (A DOS user) USER ABC 123 512K
ACCOUNT 985 USER XYZ PASSWORD CONSOLE 009 3215 ACCOUNT NUMBER BIN14 MOISK 230 2314 000050 OSDOS1 W CONSOLE 01 F 3215 MOISK 231 2314 100020 OSDOS1 W SPOOL C 2540 READER MOISK 232 2314 031 030 CPVOL 1 W SPOOL 0 2540 PUNCH SPOOL OOC 2540 READER A SPOOL E 1403 SPOOL 000 2540 PUNCH A MDISK 130 2314050050 OSOOSl W SPOOL OOE 1403 A MDISK 131 2314001020 CPVOL1 W
Note: VM/370 allows cylinders on a 2314 or 2319 normally reserved for alternate track assignment
(cylinders 200 to 202) to be optionally used for normal data storage if included within the limits of
a minidisk.
Figure 16. Use and Definition of Minidisks The real volume CPVOLl also contains disk areas assigned to userid
ABC (virtual device address 232) and userid XIZ (virtual device address
131) .
96 IBM VM/370 Planning and System Generation Guide
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
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