6.0.1

3 Determines the Current Failover Capacity of the cluster.
This is the number of hosts that can fail and still leave enough slots to satisfy all of the powered-on
virtual machines.
4 Determines whether the Current Failover Capacity is less than the Configured Failover Capacity
(provided by the user).
If it is, admission control disallows the operation.
NOTE You can set a specific slot size for both CPU and memory in the admission control section of the
vSphere HA settings in the vSphere Web Client.
Slot Size Calculation
vSphere HA Slot Size and Admission Control
(http://link.brightcove.com/services/player/bcpid2296383276001?
bctid=ref:video_vsphere_slot_admission_control)
Slot size is comprised of two components, CPU and memory.
n
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 specified 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 38.
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 settings in the vSphere Web Client. Virtual machines might require multiple slots if you have specified a
fixed 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 figures 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.
vSphere Availability
24 VMware, Inc.