When VM/370 initiates I/O operations for its own paging and spooling,
the operaticn is not subject to translation, and VM/370 itself performs
the operation.
In most cases, many virtual machines share the I/O devices and control
units on a channel both as minidisks and dedicated devices and with VM/370 system functions such as paging and spooling. Because of this
sharing, VM/370 has to schedule all the I/O requests to achieve a
balance amcng virtual machines. In addition, VM/370 must reflect the
results of the subsequent I/O interruption to the appropriate storage
areas of each virtual machine.
By specifying a dedicated channel for a virtual machine (using the
class B ATTACH CHANNEL command), the virtual machine has the channel and
all the devices for its own exclusive use. For dedicated channels, VM/370 translates the virtual storage locations specified in channel co.mands to real locations and performs any necessary paging operations.
However, VM/370 does not translate a device address because the virtual
device addresses on·the dedicated channel must match the real device
addresses; thus, minidisks cannot be usede Dedicated devices should be considered as an alternative to dedicated
channels because then the only translation done by VM/370 is device
address translation. Dedicated devices should be specified on separate
channels so that VM/370 can handle them more efficiently for a virtual Ilachine. Virtual machines can DASD as either minidisks or dedicated volumes.
Areal disk volume can be shared by several virtual machine users 6 each
owning a number of contiguous cylinders. This logical subdividing of a
real disk volume is called physical pack sharing, and each subdivision
of cylinders is called a virtual disk or minidisk. A real disk voluae can also be dedicated to a specific virtual machine for its own private
use. When using dedicated disk volumes, the virtual machine's operating
system must perform all necessary interruption handling, error recovery,
and error recording.
By using either the LINK directory control statement or the CP LINK
command, a user can share the data on a minidisk or entire disk volume
with the owner of the virtual disk. The LINK statement or command
allows controlled, concurrent access to the data on the virtual disk.
This sharing is called logical data sharing.
If any virtual machine temporarily requires additional direct access space, the user can use the CP DEFINE command to obtain it dynamically from a pool of temporary (T-disk) space. To define a pool of T-disk space, an installation specifies the size of the T-disk pool when
allocating disk space with the stand-alone CP format/allocate program. Section 1. General Considerations 17
Before using the T-disk space, the user must first initialize it.
For storing DOS, OS, or VSAM files, use the IBCDASDI program to
initialize the minidisk and set up the VTOC. For CMS files, issue the CMS FORMAT command. Temporary minidisks are available to the virtual
.achine for the duration of the current terminal session. The area is
returned to VM/310 when one of the following actions occur: The system forces the virtual machine off. The virtual machine logs off. The user issues a CP DETACH command to release the temporary
minidisk.
For details about defining, formatting, using, and sharing minidisks, refer to 18 Systems in a Virtual Machine
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