6.0.1

vSphere HA uses the actual reservations of the virtual machines. If a virtual machine does not have
reservations, meaning that the reservation is 0, a default of 0MB memory and 32MHz CPU is applied.
NOTE The Percentage of Cluster Resources Reserved admission control policy also checks that there are at
least two vSphere HA-enabled hosts in the cluster (excluding hosts that are entering maintenance mode). If
there is only one vSphere HA-enabled host, an operation is not allowed, even if there is a sufficient
percentage of resources available. The reason for this extra check is that vSphere HA cannot perform
failover if there is only a single host in the cluster.
Computing the Current Failover Capacity
The total resource requirements for the powered-on virtual machines is comprised of two components, CPU
and memory. vSphere HA calculates these values.
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The CPU component by summing the CPU reservations of the powered-on virtual machines. If you
have not specified a CPU reservation for a virtual machine, it is assigned a default value of 32MHz (this
value can be changed using the das.vmcpuminmhz advanced option.)
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The memory component by summing the memory reservation (plus memory overhead) of each
powered-on virtual machine.
The total host resources available for virtual machines is calculated by adding the hosts' CPU and memory
resources. These amounts are those contained in the host's root resource pool, not the total physical
resources of the host. Resources being used for virtualization purposes are not included. Only hosts that are
connected, not in maintenance mode, and have no vSphere HA errors are considered.
The Current CPU Failover Capacity is computed by subtracting the total CPU resource requirements from
the total host CPU resources and dividing the result by the total host CPU resources. The Current Memory
Failover Capacity is calculated similarly.
Example: Admission Control Using Percentage of Cluster Resources Reserved
Policy
The way that Current Failover Capacity is calculated and used with this admission control policy is shown
with an example. Make the following assumptions about a cluster:
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The cluster is comprised of three hosts, each with a different amount of available CPU and memory
resources. The first host (H1) has 9GHz of available CPU resources and 9GB of available memory, while
Host 2 (H2) has 9GHz and 6GB and Host 3 (H3) has 6GHz and 6GB.
n
There are five powered-on virtual machines in the cluster with differing CPU and memory
requirements. VM1 needs 2GHz of CPU resources and 1GB of memory, while VM2 needs 2GHz and
1GB, VM3 needs 1GHz and 2GB, VM4 needs 1GHz and 1GB, and VM5 needs 1GHz and 1GB.
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The Configured Failover Capacity for CPU and Memory are both set to 25%.
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 27