6.5.1

Slot size is comprised of two components, CPU and memory.
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vSphere HA calculates the CPU component by obtaining the CPU reservation of each powered-on
virtual machine and selecting the largest value. If you have not specied a CPU reservation for a virtual
machine, it is assigned a default value of 32MHz. You can change this value by using the
das.vmcpuminmhz advanced option.)
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vSphere HA calculates the memory component by obtaining the memory reservation, plus memory
overhead, of each powered-on virtual machine and selecting the largest value. There is no default value
for the memory reservation.
If your cluster contains any virtual machines that have much larger reservations than the others, they will
distort slot size calculation. To avoid this, you can specify an upper bound for the CPU or memory
component of the slot size by using the das.slotcpuinmhz or das.slotmeminmb advanced options,
respectively. See “vSphere HA Advanced Options,” on page 35.
You can also determine the risk of resource fragmentation in your cluster by viewing the number of virtual
machines that require multiple slots. This can be calculated in the admission control section of the vSphere
HA seings in the vSphere Web Client. Virtual machines might require multiple slots if you have specied a
xed slot size or a maximum slot size using advanced options.
Using Slots to Compute the Current Failover Capacity
After the slot size is calculated, vSphere HA determines each host's CPU and memory resources that are
available for virtual machines. These amounts are those contained in the host's root resource pool, not the
total physical resources of the host. The resource data for a host that is used by vSphere HA can be found on
the host's Summary tab on the vSphere Web Client. If all hosts in your cluster are the same, this data can be
obtained by dividing the cluster-level gures by the number of hosts. Resources being used for
virtualization purposes are not included. Only hosts that are connected, not in maintenance mode, and that
have no vSphere HA errors are considered.
The maximum number of slots that each host can support is then determined. To do this, the host’s CPU
resource amount is divided by the CPU component of the slot size and the result is rounded down. The
same calculation is made for the host's memory resource amount. These two numbers are compared and the
smaller number is the number of slots that the host can support.
The Current Failover Capacity is computed by determining how many hosts (starting from the largest) can
fail and still leave enough slots to satisfy the requirements of all powered-on virtual machines.
Example: Admission Control Using Slot Policy
The way that slot size is calculated and used with this admission control policy is shown in an example.
Make the following assumptions about a cluster:
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The cluster is comprised of three hosts, each with a dierent amount of available CPU and memory
resources. The rst 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.
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There are ve powered-on virtual machines in the cluster with diering 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 Host Failures Cluster Tolerates is set to one.
vSphere Availability
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