6.0.1

Best Practices for Admission Control
Observe the following best practices for the configuration and usage of admission control for vSphere HA.
The following recommendations are best practices for vSphere HA admission control.
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Select the Percentage of Cluster Resources Reserved admission control policy. This policy offers the
most flexibility in terms of host and virtual machine sizing. When configuring this policy, choose a
percentage for CPU and memory that reflects the number of host failures you want to support. For
example, if you want vSphere HA to set aside resources for two host failures and have ten hosts of
equal capacity in the cluster, then specify 20% (2/10).
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Ensure that you size all cluster hosts equally. For the Host Failures Cluster Tolerates policy, an
unbalanced cluster results in excess capacity being reserved to handle failures because vSphere HA
reserves capacity for the largest hosts. For the Percentage of Cluster Resources Policy, an unbalanced
cluster requires that you specify larger percentages than would otherwise be necessary to reserve
enough capacity for the anticipated number of host failures.
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If you plan to use the Host Failures Cluster Tolerates policy, try to keep virtual machine sizing
requirements similar across all configured virtual machines. This policy uses slot sizes to calculate the
amount of capacity needed to reserve for each virtual machine. The slot size is based on the largest
reserved memory and CPU needed for any virtual machine. When you mix virtual machines of
different CPU and memory requirements, the slot size calculation defaults to the largest possible, which
limits consolidation.
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If you plan to use the Specify Failover Hosts policy, decide how many host failures to support and then
specify this number of hosts as failover hosts. If the cluster is unbalanced, the designated failover hosts
should be at least the same size as the non-failover hosts in your cluster. This ensures that there is
adequate capacity in case of failure.
Best Practices for Cluster Monitoring
Observe the following best practices for monitoring the status and validity of your vSphere HA cluster.
Setting Alarms to Monitor Cluster Changes
When vSphere HA or Fault Tolerance take action to maintain availability, for example, a virtual machine
failover, you can be notified about such changes. Configure alarms in vCenter Server to be triggered when
these actions occur, and have alerts, such as emails, sent to a specified set of administrators.
Several default vSphere HA alarms are available.
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Insufficient failover resources (a cluster alarm)
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Cannot find master (a cluster alarm)
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Failover in progress (a cluster alarm)
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Host HA status (a host alarm)
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VM monitoring error (a virtual machine alarm)
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VM monitoring action (a virtual machine alarm)
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Failover failed (a virtual machine alarm)
NOTE The default alarms include the feature name, vSphere HA.
Monitoring Cluster Validity
A valid cluster is one in which the admission control policy has not been violated.
Chapter 2 Creating and Using vSphere HA Clusters
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