6.0.1

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 different hosts.
When vSphere HA performs failover and restarts virtual machines on different hosts, its first 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 off
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.
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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 insufficient powered-on capacity to perform a failover.
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VM-Host affinity (required) rules might limit the hosts on which certain virtual machines can be placed.
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There might be sufficient 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 confirm host power-on recommendations. Similarly, if DRS is
in manual mode, you might need to confirm migration recommendations.
If you are using VM-Host affinity 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.
vSphere HA and DRS Affinity Rules
If you create a DRS affinity rule for your cluster, you can specify how vSphere HA applies that rule during a
virtual machine failover.
The two types of rules for which you can specify vSphere HA failover behavior are the following:
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VM anti-affinity rules force specified virtual machines to remain apart during failover actions.
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VM-Host affinity rules place specified virtual machines on a particular host or a member of a defined
group of hosts during failover actions.
When you edit a DRS affinity rule, select the checkbox or checkboxes that enforce the desired failover
behavior for vSphere HA.
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HA must respect VM anti-affinity rules during failover -- if VMs with this rule would be placed
together, the failover is aborted.
Chapter 2 Creating and Using vSphere HA Clusters
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