6.7

Host limits In addition to resource reservations, a virtual machine can only be placed
on a host if doing so does not violate the maximum number of allowed
virtual machines or the number of in-use vCPUs.
Feature constraints If the advanced option has been set that requires vSphere HA to enforce
VM to VM anti-affinity rules, vSphere HA does not violate this rule. Also,
vSphere HA does not violate any configured per host limits for fault tolerant
virtual machines.
If no hosts satisfy the preceding considerations, the master host issues an event stating that there are not
enough resources for vSphere HA to start the VM and tries again when the cluster conditions have
changed. For example, if the virtual machine is not accessible, the master host tries again after a change
in file accessibility.
VM and Application Monitoring
VM Monitoring restarts individual virtual machines if their VMware Tools heartbeats are not received
within a set time. Similarly, Application Monitoring can restart a virtual machine if the heartbeats for an
application it is running are not received. You can enable these features and configure the sensitivity with
which vSphere HA monitors non-responsiveness.
When you enable VM Monitoring, the VM Monitoring service (using VMware Tools) evaluates whether
each virtual machine in the cluster is running by checking for regular heartbeats and I/O activity from the
VMware Tools process running inside the guest. If no heartbeats or I/O activity are received, this is most
likely because the guest operating system has failed or VMware Tools is not being allocated any time to
complete tasks. In such a case, the VM Monitoring service determines that the virtual machine has failed
and the virtual machine is rebooted to restore service.
Occasionally, virtual machines or applications that are still functioning properly stop sending heartbeats.
To avoid unnecessary resets, the VM Monitoring service also monitors a virtual machine's I/O activity. If
no heartbeats are received within the failure interval, the I/O stats interval (a cluster-level attribute) is
checked. The I/O stats interval determines if any disk or network activity has occurred for the virtual
machine during the previous two minutes (120 seconds). If not, the virtual machine is reset. This default
value (120 seconds) can be changed using the advanced option das.iostatsinterval.
To enable Application Monitoring, you must first obtain the appropriate SDK (or be using an application
that supports VMware Application Monitoring) and use it to set up customized heartbeats for the
applications you want to monitor. After you have done this, Application Monitoring works much the same
way that VM Monitoring does. If the heartbeats for an application are not received for a specified time, its
virtual machine is restarted.
You can configure the level of monitoring sensitivity. Highly sensitive monitoring results in a more rapid
conclusion that a failure has occurred. While unlikely, highly sensitive monitoring might lead to falsely
identifying failures when the virtual machine or application in question is actually still working, but
heartbeats have not been received due to factors such as resource constraints. Low sensitivity monitoring
results in longer interruptions in service between actual failures and virtual machines being reset. Select
an option that is an effective compromise for your needs.
vSphere Availability
VMware, Inc. 16