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you only want a single end-to-end process to react to it; the last thing you want is for two different
technologies, such as VMware's HA feature and Windows Failover Clustering to respond to the
same issue at the same time.
MANAGING CONTENTION
In looking at some of reasons for virtualization's popularity, the preceding sections identii ed the
concept of contention, the capability to better use previously underutilized physical resources in a
server in order to reduce the total number of physical servers deployed. For the purposes of this
discussion, we can split the idea of contention into two parts: good contention and bad contention.
Good Contention
Good contention is straightforward: It enables you to see positive benei ts from virtualizing your
servers, ultimately resulting in less time and money spent on deploying and maintaining your
physical server estate.
For example, if the average CPU utilization of 6 single CPU physical servers was 10% and none of
them had concurrent peak CPU usage periods, then I would feel comfortable virtualizing those 6
servers and running them as a single server with a single CPU — the logic being 6
3
5
60%,
and therefore less than the capacity of a single server with a single CPU. I'd want to make sure there
was sufi cient physical memory and storage system performance available for all 6 virtual servers,
but ultimately the benei t would be the ability to retire 5 physical servers.
10%
That's a very simple example but one that most businesses can readily understand. CPU utilization is
an absolute number that is usually a good rel ection of how busy the server is. Conversely, sizing the
server's memory is something to which you can't apply such an easy consolidation methodology to.
Instead, you usually need to determine the total memory requirement of all the virtual servers you
want to run on a host server and then ensure you have more than that amount of physical memory
in the host. However, VMware's hypervisor complicates that by offering a memory de-duplication
feature that allows duplicate memory pages to be replaced with a link to a single memory page shared
by several virtual servers, but over-estimating the benei t this technology could deliver wrong can
result in the performance issues you tried to avoid. For SQL Server environments that are dependent
on access to large amounts of physical memory, trusting these hypervisor memory consolidation
technologies still requires testing, so their use in sizing exercises should be minimized.
Bad Contention
Not all contention is good. In fact, unless you plan well you're more likely to have bad contention
than good contention. To understand bad contention, consider the CPU utilization example from
the preceding section: 6 servers with average CPU utilization values of 10% being consolidated onto
a single CPU host server. This resulted in an average CPU utilization for the host server of around
60%. Now imagine if the average CPU utilization for two of the virtual servers jumps from 10% to
40%. As a consequence, the total CPU
requirement
has increased from 60% to 120%. Obviously,
the total CPU utilization cannot be 120%, so you have a problem. Fortunately, resolving this
scenario is one of the core functions of hypervisor software: How can it look like CPU utilization is
120%, for example, when actually only 100% is available?
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