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6: A Most-Significant Step—“Virtualizing” Your IT Systems 93

VMFS cluster file system allows both the source and the target server to access
the virtual machine files concurrently. The memory and execution state of a
virtual machine can then be transmitted over a high-speed network. The net-
work is also virtualized by VMware ESX, so the virtual machine retains its
network identity and connections, ensuring a seamless migration process.
IBM’s System p Live Partition Mobility offers a similar concept.
Xen uses either the paravirtualization approach (as the POWER™ archi-
tecture does) or full virtualization. In the partial approach (paravirtualiza-
tion), virtualized operating systems should be virtual-aware. Xen, for
example, requires virtual Linux systems to run a modified Linux kernel. Such
an approach establishes restrictions to the usable operating systems.
However, although they are hypervisor-aware, different operating systems
with their application stacks can be active on one machine. In the full
approach, the hardware, such as Intel’s Vanderpool or AMD’s Pacifica tech-
nology, must be virtual-aware. In this case, running unmodified guests on
top of the Xen hypervisor is possible, gaining the speed of the hardware.
Another technique is operating system level virtualization. One operating
system on a machine is capable of making virtual instances of itself available
ptg
as a virtual system. Solaris containers (or zones) are an example of this tech-
nique. In contrast to the other techniques, all virtualized systems run on the
same operating system level, which is the only operating system the machine
provides. This can become a limiting restriction, especially when consolidat-
ing different server generations. Often the application stack is heavily
dependent on the particular operating system. We reach a dead end when we
want to consolidate servers running different operating systems such as
Windows and Linux.

Storage Virtualization

Computer systems are not the only candidates for virtualizing; storage can
be virtualized, too. This section describes IBM SAN Volume Controller,
which provides a virtual pool of storage consisting of SAN-attached physical
storage devices.

IBM SAN Volume Controller

The SAN Volume Controller (SVC) is a hardware device that brings
storage devices in a SAN together in a virtual pool. This makes your storage
appear as one logical device to manage. To the connected computers, SVC