6.0.1

vSphere HA Admission Control
vCenter Server uses admission control to ensure that sufficient resources are available in a cluster to provide
failover protection and to ensure that virtual machine resource reservations are respected.
Three types of admission control are available.
Host
Ensures that a host has sufficient resources to satisfy the reservations of all
virtual machines running on it.
Resource Pool
Ensures that a resource pool has sufficient resources to satisfy the
reservations, shares, and limits of all virtual machines associated with it.
vSphere HA
Ensures that sufficient resources in the cluster are reserved for virtual
machine recovery in the event of host failure.
Admission control imposes constraints on resource usage and any action that would violate these
constraints is not permitted. Examples of actions that could be disallowed include the following:
n
Powering on a virtual machine.
n
Migrating a virtual machine onto a host or into a cluster or resource pool.
n
Increasing the CPU or memory reservation of a virtual machine.
Of the three types of admission control, only vSphere HA admission control can be disabled. However,
without it there is no assurance that the expected number of virtual machines can be restarted after a failure.
Do not permanently disable admission control, however you might need to do so temporarily, for the
following reasons:
n
If you need to violate the failover constraints when there are not enough resources to support them--for
example, if you are placing hosts in standby mode to test them for use with Distributed Power
Management (DPM).
n
If an automated process needs to take actions that might temporarily violate the failover constraints (for
example, as part of an upgrade or patching of ESXi hosts as directed by vSphere Update Manager).
n
If you need to perform testing or maintenance operations.
Admission control sets aside capacity, but when a failure occurs vSphere HA uses whatever capacity is
available for virtual machine restarts. For example, vSphere HA places more virtual machines on a host than
admission control would allow for user-initiated power ons.
NOTE When vSphere HA admission control is disabled, vSphere HA ensures that there are at least two
powered-on hosts in the cluster even if DPM is enabled and can consolidate all virtual machines onto a
single host. This is to ensure that failover is possible.
Host Failures Cluster Tolerates Admission Control Policy
You can configure vSphere HA to tolerate a specified number of host failures. With the Host Failures
Cluster Tolerates admission control policy, vSphere HA ensures that a specified number of hosts can fail
and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.
With the Host Failures Cluster Tolerates policy, vSphere HA performs admission control in the following
way:
1 Calculates the slot size.
A slot is a logical representation of memory and CPU resources. By default, it is sized to satisfy the
requirements for any powered-on virtual machine in the cluster.
2 Determines how many slots each host in the cluster can hold.
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 23