Information Technology Reference
In-Depth Information
Configuring vSphere High Availability VM Options
Figure 7.19 shows the VM options that are available to control the behavior of VMs for vSphere
HA. Two VM options are available for administrators to coni gure: VM Restart Priority and
Host Isolation Response. Both options are coni gurable as a cluster default setting as well as a
per-VM setting.
Figure 7.19
You can defi ne
cluster default VM
options to custom-
ize the behavior of
vSphere HA.
vSphere High Availability VM Restart Priority
Not all VMs are equal. Some VMs are more important or more critical and require higher prior-
ity when ensuring availability. When an ESXi host experiences failure and the remaining cluster
nodes are tasked by vSphere HA with bringing VMs back online, they have a i nite amount of
resources before there are no more resources to allocate to VMs that need to be powered on.
This is especially true when Admission Control is set to Disabled, allowing more VMs to be
powered on than the cluster could support given a failure. Rather than leave important VMs
to chance, a vSphere HA-enabled cluster allows you to prioritize VMs through VM Restart
Priority.
The VM Restart Priority options for VMs in a vSphere HA-enabled cluster include Low,
Medium, High, and Disabled. For VMs that should be brought up i rst, the restart priority
should be set to High. For VMs that should be brought up if resources are available, the restart
priority can be set to Medium or Low. For VMs that will not be missed for a period of time and
should not be brought online when available resources are low, the restart priority should be set
to Disabled. You can dei ne a default restart priority for the entire cluster, as shown in
Figure 7.19, but what if there is a VM that is more (or less) important? The VM Overrides sec-
tion allows you to dei ne a per-VM restart priority. Figure 7.20 shows VM Restart Priority set
to Medium for the cluster and set to low for another VM based on their importance to the
organization.
The restart priority is put into place only for the VMs running on the ESXi hosts that experi-
ence an unexpected failure. VMs running on hosts that have not failed are not affected by the
restart priority. It is possible then that VMs coni gured with a restart priority of High might not
be powered on by vSphere HA because of limited resources, which is in part because of lower-
priority VMs that continue to run (again, only if Admission Control was set to Disabled). For
