Information Technology Reference
In-Depth Information
The opposite problem is also possible, although it is typically viewed as an
opportunity for improved efficiency: An existing workload does not use all of a
physical computer's resources, or its demand on the system has shrunk over time.
This situation offers an opportunity to host another workload on the same system
(consolidation), requiring workload isolation via virtualization.
The ideal solution is an elastic computer—one that can automatically and im-
mediately grow or shrink to meet the workload's needs. No computer exists that
can be scaled infinitely up or down. A growing workload needs a larger computer;
a shrinking workload leaves room for another workload. Sometimes a growing
workload can be partitioned into smaller workloads, but partitioning requires a
service outage. Even when that is possible, at least one portion of the original
workload must be moved to a different system.
A critical facet of consolidation, and therefore of virtualization, is resource
management. Resource controls must be available to prevent one workload from
using so much system capacity—compute capacity, RAM, or I/O capacity—that
other workloads suffer. Virtualization can popularize and extend existing methods
of resource control and can also enable a new solution: workload mobility.
Today's virtualization technologies go beyond the traditional consolidated system
to offer new, exciting opportunities. They can be categorized based on their focus.
Some technologies provide business benefits by enhancing business agility and
operational flexibility, which are related in some ways. Others achieve business
continuity, which is largely a different topic. The next sections explore each of
these possibilities.
9.2.2.1 Operational Flexibility
Operational flexibility is the ability to change one or more properties of the envi-
ronment in which the workload is running. It is the domain of data center teams,
and accommodates smaller changes in resource demands. Nevertheless, these
changes, if ignored, can affect the response times of that workload or of others.
Resource controls are often used to limit the ability of one workload to consume
system resources. They are needed for consolidated systems, to ensure that all
workloads have access to appropriate resource capacity.
Imagine a workload that normally requires two CPUs to achieve its desired
response time; during the last week of each fiscal quarter, however, it needs four
CPUs to complete its tasks. If it is the sole workload on the system, the system
must have four CPUs, but two of them will be idle for most of the quarter, wasting
the investment in them. In contrast, if this workload is found in one of a few VEs
on a larger, eight-CPU system, a resource control can limit the workload to two
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