Minidisks
In addition, all virtual machine minidisks should have a
virtual cylinder 0, track 0, record 3. Labels created by IEHDASDR, or INTDK
label at IBCDASDI, VOL1xxxxxx where xxxxxx is a 6-character volume label.
A physical volume that holds only virtual machine minidisks can have
the first of those minidisks starting at real cylinder O. CP recognizes
the physical volume if the first minidisk has a valid label.
In Figure 16, the volume indicated as OSDOS1 has
allocated to a minidisk that is formatted for use
serial number of that minidisk must be OSDOS1, associated with the real volume. Since the minidisk
physical volume, changing it affects the directory
who have minidisks on that volume.
its real cylinder 0 by os. The volume
the label that is
label identifies the
entries of all users You should not assign real cylinder
because that user (if he has read/write
the label on the minidisk.
o to a user as a data area,
access to the disk) can rewrite
Additionally, you must not assign user minidisks to begin on real
cylinder 0 of any physical volumes that are to contain CP controlled
areas (for paging, spooling, and so on). On these volumes, cylinder 0 track 0 record 4 contains control information required by CP. The VTOC labels written are compatible with OS, but indicate to OS that there is
no space on that DASD. The initialization programs used to format OS, DOS, and CMS/VSAM minidisks write over and destroy this necessary
control information if the space is assigned to a user minidisk, and
this causes CP system failures.
Sharing Minidisks
A minidisk can be shared by multiple virtual machines. One virtual
machine is designated the owner of the minidisk (it has an MDISK control
statement in its VM/370 directory entry describing the minidisk) and
other virtual machines can link to the minidisk.
For example, assume a virtual machine called USERA owns a minidisk at
address 150. The VM/370 directory entry for USER! contains the
following statement: MDISK 150 3330 050 010 SYS003 W READPASS USERA's virtual disk is on the volume labeled SYS003 and occupies real
cylinders 050-059. Any other virtual machine that issues the CP LINK command with the
proper password, or has the following LINK statement in its VM/370 directory entry, can read the 150 minidisk belonging to USER!. LINK USERA 150 cuu RR
The cuu is the virtual device address at which the 150 minidisk
belonging to USERA is linked to another virtual machine. If you define
another virtual machine, USERB, with the following statement in its VM/3?0 directory entry:
LINK USERA 150 151 RR Part 1. Planning for System Generation 101
Minidisks USERB can read data from USERA's 150 virtual disk whenever it issues a
read for data on its own 151 virtual disk.
You can link to any minidisk that is defined in the V8/370 directory
if that minidisk has a read and/or write password specified in the MDISK control statement and if the type of link you desire is allowed. Three
types of sharing may exist and, correspondingly, three passwords may be
specified in the MDISK record.
Minidisks may be shared in the following ways: • Read-only (R) indicates that all virtual machines sharing the disk
are usin9 it in read status. • Read/write (W) indicates that one virtual machine may have read/write
access and multiple virtual machines may have concurrent read-only
access. • Multi-write (MW) indicates that multiple virtual machines may issue
writes concurrently to the disk. Generally: of access requires that the virtual machines include code to control this, such
as the shared DASD support of os. See the description of the CP LINK !Qr for more
to minidisks.
command in the !M/370 £f information about linking 102 IBM VM/370 Planning and System Generation Guide
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