6.5.1

Capacity Reservation Settings
When you reserve capacity for your vSphere HA cluster with an admission control policy, you must
coordinate this seing with the corresponding vSAN seing that ensures data accessibility on failures.
Specically, the Number of Failures Tolerated seing in the vSAN rule set must not be lower than the
capacity that the vSphere HA admission control seing reserved.
For example, if the vSAN rule set allows for only two failures, the vSphere HA admission control policy
must reserve capacity that is equivalent to only one or two host failures. If you are using the Percentage of
Cluster Resources Reserved policy for a cluster that has eight hosts, you must not reserve more than 25% of
the cluster resources. In the same cluster, with the Host Failures Cluster Tolerates policy, the seing must not
be higher than two hosts. If vSphere HA reserves less capacity, failover activity might be unpredictable.
Reserving too much capacity overly constrains the powering on of virtual machines and intercluster
vSphere vMotion migrations.
Using vSphere HA and DRS Together
Using vSphere HA with Distributed Resource Scheduler (DRS) combines automatic failover with load
balancing. This combination can result in a more balanced cluster after vSphere HA has moved virtual
machines to dierent hosts.
When vSphere HA performs failover and restarts virtual machines on dierent hosts, its rst priority is the
immediate availability of all virtual machines. After the virtual machines have been restarted, those hosts on
which they were powered on might be heavily loaded, while other hosts are comparatively lightly loaded.
vSphere HA uses the virtual machine's CPU and memory reservation and overhead memory to determine if
a host has enough spare capacity to accommodate the virtual machine.
In a cluster using DRS and vSphere HA with admission control turned on, virtual machines might not be
evacuated from hosts entering maintenance mode. This behavior occurs because of the resources reserved
for restarting virtual machines in the event of a failure. You must manually migrate the virtual machines o
of the hosts using vMotion.
In some scenarios, vSphere HA might not be able to fail over virtual machines because of resource
constraints. This can occur for several reasons.
n
HA admission control is disabled and Distributed Power Management (DPM) is enabled. This can
result in DPM consolidating virtual machines onto fewer hosts and placing the empty hosts in standby
mode leaving insucient powered-on capacity to perform a failover.
n
VM-Host anity (required) rules might limit the hosts on which certain virtual machines can be placed.
n
There might be sucient aggregate resources but these can be fragmented across multiple hosts so that
they can not be used by virtual machines for failover.
In such cases, vSphere HA can use DRS to try to adjust the cluster (for example, by bringing hosts out of
standby mode or migrating virtual machines to defragment the cluster resources) so that HA can perform
the failovers.
If DPM is in manual mode, you might need to conrm host power-on recommendations. Similarly, if DRS is
in manual mode, you might need to conrm migration recommendations.
If you are using VM-Host anity rules that are required, be aware that these rules cannot be violated.
vSphere HA does not perform a failover if doing so would violate such a rule.
For more information about DRS, see the vSphere Resource Management documentation.
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 25