6.0.1

If a master host is unable to communicate directly with the agent on a slave host, the slave host does not
respond to ICMP pings, and the agent is not issuing heartbeats it is considered to have failed. The host's
virtual machines are restarted on alternate hosts. If such a slave host is exchanging heartbeats with a
datastore, the master host assumes that it is in a network partition or network isolated and so continues to
monitor the host and its virtual machines. See “Network Partitions,” on page 21.
Host network isolation occurs when a host is still running, but it can no longer observe traffic from vSphere
HA agents on the management network. If a host stops observing this traffic, it attempts to ping the cluster
isolation addresses. If this also fails, the host declares itself as isolated from the network.
The master host monitors the virtual machines that are running on an isolated host and if it observes that
they power off, and the master host is responsible for the virtual machines, it restarts them.
NOTE If you ensure that the network infrastructure is sufficiently redundant and that at least one network
path is available at all times, host network isolation should be a rare occurrence.
Determining Responses to Host Issues
If a host fails and its virtual machines must be restarted, you can control the order in which the virtual
machines are restarted with the VM restart priority setting. You can also configure how vSphere HA
responds if hosts lose management network connectivity with other hosts by using the host isolation
response setting. Other factors are also considered when vSphere HA restarts a virtual machine after a
failure.
The following settings apply to all virtual machines in the cluster in the case of a host failure or isolation.
You can also configure exceptions for specific virtual machines. See “Customize an Individual Virtual
Machine,” on page 40.
VM Restart Priority
VM restart priority determines the relative order in which virtual machines are allocated resources after a
host failure. Such virtual machines are assigned to hosts with unreserved capacity, with the highest priority
virtual machines placed first and continuing to those with lower priority until all virtual machines have
been placed or no more cluster capacity is available to meet the reservations or memory overhead of the
virtual machines. A host then restarts the virtual machines assigned to it in priority order. If there are
insufficient resources, vSphere HA waits for more unreserved capacity to become available, for example,
due to a host coming back online, and then retries the placement of these virtual machines. To reduce the
chance of this situation occurring, configure vSphere HA admission control to reserve more resources for
failures. Admission control allows you to control the amount of cluster capacity that is reserved by virtual
machines, which is unavailable to meet the reservations and memory overhead of other virtual machines if
there is a failure.
The values for this setting are Disabled, Low, Medium (the default), and High. The Disabled setting is
ignored by the vSphere HA VM/Application monitoring feature because this feature protects virtual
machines against operating system-level failures and not virtual machine failures. When an operating
system-level failure occurs, the operating system is rebooted by vSphere HA, and the virtual machine is left
running on the same host. You can change this setting for individual virtual machines.
NOTE A virtual machine reset causes a hard reboot of the guest operating system, but does not power cycle
the virtual machine.
The restart priority settings for virtual machines vary depending on user needs. Assign higher restart
priority to the virtual machines that provide the most important services.
For example, in the case of a multitier application, you might rank assignments according to functions
hosted on the virtual machines.
n
High. Database servers that provide data for applications.
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 15