IBM ZVM - FOR LINUX V6 RELEASE 1, Getting Started

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Each virtual machine has its own virtual storage. CP manages the residency of
virtual machine’s pages in real storage through paging. Pages that have not been
referenced can be moved out of real storage into either expanded storage or onto a
paging device. When a virtual machine requires a page no longer in real storage, a
page fault occurs and CP brings the missing page back into real storage.
CP has facilities that allow portions of real storage to be shared by many virtual
machines. Such portions are called
shared segments . This sharing economizes on real
storage and requires less paging, thereby improving performance. For example, the
CMS nucleus is shared in real storage by all virtual machines that loaded CMS by
name; that is, every CMS virtual machine maps a 1 MB segment of virtual storage
to the same 1 MB of real storage.
DASD and minidisks
DASD, the mainframe term for disk drives, stands for “direct access storage
device” and is analogous to a hard disk drive on a personal computer. A single
real DASD is called a
volume or real volume . Each volume has a label or volume serial
number (volser) that identifies the volume to z/VM.
Of special importance is the way z/VM shares DASD. CP can logically partition
real DASD volumes into
minidisks , which is analogous to dividing a personal
computer hard disk into multiple partitions. A minidisk has its own label, which is
distinct from the real DASD label. Each virtual machine can have one or more
minidisks and those minidisks are under control of the guest operating system. To
the guest, a minidisk appears as an entire DASD volume (though smaller) and the
guest runs channel programs as normal to do I/O. Behind the scenes, CP reorients
the channel programs: the guest perceives all minidisks as starting at cylinder 0,
but the real DASD volume has only one cylinder 0, so CP must modify the
cylinder offsets in the channel program to address DASD cylinders owned by the
guest.
Temporary minidisks
You can create a temporary minidisk from a special pool of real disks. The disk
lasts as long as the virtual machine is logged on. At logoff, the temporary minidisk
is deleted and the space returned to the available temporary disk pool.
Virtual disks in storage
Virtual disks in storage are similar to temporary minidisks, except the disks are
mapped to storage rather than the cylinders of real disks. Using virtual disks in
storage avoids the need for disk I/O. CP manages the virtual disk pages as part of
its real memory management.
Virtual readers, punches, and printers
These devices are not associated with real devices, but are implemented through
the spool file system. For more information, see “Overview of the CP spool file
system” on page 7.
The virtual machine console
The virtual machine console or virtual console is the primary interface to the virtual
machine. When you log on to a virtual machine from a local terminal or a remote
workstation, the virtual console is associated with the terminal session. From the
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z/VM: Getting Started with Linux on System z