Information Technology Reference
In-Depth Information
Although the applications function correctly, their performance characteristics
will probably change when they are running in a consolidated environment. A
variety of factors may cause this change:
Shared I/O channels are typical in most virtualization implementations, and
will reduce available bandwidth to networks and persistent storage.
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Disks may be shared, increasing read and write transaction latency.
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If there are more workloads than CPUs, workloads may wait longer before
getting a time slice on a CPU, thereby increasing response time.
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If multiple workloads share a set of physical CPUs and there are more work-
loads than CPUs, there is a greater chance of a process being assigned to a
different CPU than the previous time slice, negating the benefits of a CPU's
caches.
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In addition to the potential for decreased performance, workload performance
is less consistent in a consolidated environment unless resource controls are used.
During one day, two workloads might have their daily peaks in CPU utilization at
different times, and response times may be similar to those observed on unconsol-
idated systems. The next day, the peaks for those two workloads might coincide,
leading to competition for physical resources such as CPUs and RAM. Response
times for each workload may then suffer.
In other words, aggregate CPU usage of one workload should not change be-
cause of consolidation, but the deviation from average response time may increase.
However, a complete understanding of the performance demands of the workloads
should allow you to minimize the deviation because you will be able to place VEs
on different systems to minimize this kind of performance deviation.
Resource partitioning technologies, including some virtual machine implemen-
tations, dedicate hardware resources to each resource partition. This practice pro-
vides insulation for each partition from the CPU consumption of other domains.
Yo u c a n u s e t h i s f e a t u r e t o i m p r o v e p e r f o r m a n c e c o n s i s t e n c y.
Besides the effects of workload consolidation, virtualization can cause changes
in workload performance. Many virtualization technologies require use of the sys-
tem CPUs to perform virtualization activities. The result is performance overhead:
CPU cycles that are not performing work directly related to the applications. The
amount of overhead depends on several factors, including the following:
The method of virtualization (e.g., partitioning, virtual machines, or operat-
ing system virtualization):
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A partitioned system does not run virtualization software and has no
overhead.
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