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Latency-sensitive workloads that cause such performance overhead should not
be installed on systems without testing to ensure they can still meet their perfor-
mance objectives.
VE scalability is a related performance topic. Some types of virtualization can
host only a limited number of VEs. Most can theoretically run dozens or hun-
dreds of VEs. In practice, however, most of those do not scale well: They may need
terabytes of RAM, they may be limited by the scalability limits of the hardware
architecture to which they are tied, or their current implementation may create so
much performance overhead that they can scale to only 10-20 VEs. Others can ef-
fectively handle more than 100 VEs. This effect is often mitigated by the common
practice of consolidating physical systems from old, slow, low-utilization platforms
onto new processors. In such circumstances, the modest resource requirements
and the difference in processor speed may compensate for the possible overhead.
To maximize the cost savings achieved from consolidation, choose the most scal-
able virtualization solution.
Many workloads change their resource requirements over short or long periods
of time. The ability to dynamically change the amount of resources available to
those VEs is necessary to maintain appropriate throughput and response times.
Most applications are able to run on more than one version of an operating
system (OS). Some are tied to a specific version or to a specific version of an OS
package or a patch level. Such applications can benefit from the freedom to run
different VEs at different OS levels, thereby permitting applications with differ-
ent OS requirements to coexist on the same server. Also, VEs with independent
OS levels can use different patch windows for updating system software. Virtual
environments that share a common OS software level may need a synchronized
patch window. Unfortunately, this strategy may be difficult to realize when differ-
ent applications and different application owners share the same physical system.
Similarly, while many applications can run on different operating systems, oth-
ers are available only for a particular OS. The ability to run different operating
systems at the same time (e.g., Microsoft Windows and Apple MacOS) also has
value, and in some situations is an absolute necessity.
Some virtualization features enhance business agility and operational flex-
ibility—key considerations that are discussed in Chapter 9, “Virtualization
Management.” Dynamic data centers, whose workloads regularly grow and shrink,
benefit the most from these features. Cloning is the ability to create an exact copy
of a VE. It accelerates provisioning when many similar workloads will exist, and
can even be done as a programmed response to a request for a new instance of an
application. VE mobility can be used to move an application from one computer to
another. This capability is very valuable in dynamic data centers whose workloads
grow and shrink regularly, and it facilitates ongoing application availability dur-
ing physical server hardware maintenance.
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