Minidisks
Minidisks
The external storage requirements of multiple virtual machines executing
concurrently would be excessive if each virtual machine were assigned
one real direct access storage device for each virtual DASD specified in
its configuration.
Therefore, if you do not require the full capacity of a real DASD,
you can be assigned one or more minidisks instead. A minidisk is a
logical subdivision of a physical disk pack with its own virtual device
address, virtual cylinders (starting with 0, 1, 2, and so on) and a VTOC
(volume table of contents or disk label identifier). Each of your
minidisks is preallocated the number of contiguous full cylinders that
were specified in the VM/370 MDISK directory record, and that space is
considered to be a complete virtual disk device.
Minidisks are controlled and managed by CPa If a virtual machine
attempts to use DASD space beyond the boundaries defined for its
minidisks, CP presents a command reject (seek check) to the virtual
machine. If a system is to be run on both a virtual and a real machine,
minidisks for that system must start at real cylinder zero. For a
detailed list of minidisk restrictions, see "Appendix F: V"/370 Restrict ions. " The remainder of this section describes the following characteristics
of minid isks: • Definition • Space allocation • Track characteristics • Alternate tracks • Labels
O f"" M" "d" k 9_lnlng Inl IS S Permanent minidisks are defined in the VM/370 directory entry for a
virtual machine. A minidisk defined in the directory via an "DISK statement is a permanent part of the virtual machine configuration and
the data on the minidisk is available to the user from session to
session.
If any virtual machine has a temporary requirement for direct access
space, this can be filled from a pool of T-disk space. You specify the
size of the T-disk pool when you allocate disk space with the standalone
Format/Allocate program. Minidisks created from the T-disk area must be
initialized and are available to the virtual machine for the duration of
one terminal session. When the virtual machine logs off or issues a CP command to release the temporary minidisk, the area is returned to CPa It is up to you to allocate minidisks on V"/370 disks in a manner
that minimizes arm contention and physical overlap. Information about
minimizing arm contention is found in the "Preparing the CP System Control File section of Part 2. The V"/370 directory function neither checks nor flags overlapped
or duplicate minidisk extents. Nor does the function provide DASD space
records for unused space or used space. Part 1. Planning for System Generation 95

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