Datasheet

Chapter 1: Overview of Virtualization
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Increased Uptime and Faster Failure Recovery
As mentioned in the previous section, increasing the isolation of virtual machines from specific physical
hardware increases system availability by increasing the portability of those virtual machines. The
portability of virtual machines enables them to be migrated from one physical server to another if
hardware problems arise on the first system. Xen virtual machines can be migrated from one physical
host to another without any interruption in availability — the migration process is transparent to users
as well as to any processes running on those virtual machines.
Adopting virtualization and a strategy for automated problem detection and virtual machine migration
can lower the costs that are traditionally associated with redundancy and failover because much of the
hardware that was formerly required to ensure availability by having redundant physical systems can
now be provided by being able to migrate multiple virtual machines to other, suitable hardware
platforms in the event of emerging problems. You can migrate virtual systems without interrupting
service, and can physically increase availability during power failures by reducing the load on your
uninterruptible power supplies because they are supporting fewer physical machines, enabling you to
maintain the same level of system availability for a longer period.
When partitioning and deploying software and services for high availability, one key to high availability
is to efficiently divide physical and virtual machines in terms of the services that they provide. For
example, in a completely virtualized environment, the primary purpose of your physical machines
should be to support your virtual machines; they should not provide any external software services
themselves. This enables you to respond to emerging hardware problems on your physical hardware by
migrating your virtual machines to other physical hardware without having to worry about any
software services that are provided by the physical machines themselves (other than support for Xen, of
course). For example, you do not want to both run a Web server and support virtual machines on a
physical system if you can avoid it because the failure of that physical system will make the Web server
unavailable even after you have successfully migrated your virtual machines to other physical hosts. In
general, you want to keep your IT infrastructure as independent as possible of the physical systems on
which any portion of it is currently executing.
Simplified Capacity Expansion
Virtualization solutions such as virtual machines and storage virtualization remove the hard limits that
are often imposed by physical machines or local-storage solutions. Virtual machines can be moved from
one physical piece of hardware to another to enable them to benefit from hardware improvements, such
as more powerful CPUs, additional CPU cores, additional memory, additional or faster network cards,
and so on. Similarly, storage virtualization makes it possible to transparently increase the amount
of available storage and the size of existing partitions and filesystems.
Simpler Support for Legacy Systems and Applications
Virtualization is an excellent solution to the need to run legacy software. Many businesses have certain
applications that they depend on, but which may no longer be available from a specific vendor or which
may not yet have been upgraded so that they can run on newer operating systems or hardware.
Although depending on old software that itself depends on a specific version of an operating system is
problematic from a business standpoint, it still may be a business reality.
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