6.5.1

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-anity rules, vSphere HA does not violate this rule. Also,
vSphere HA does not violate any congured 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 le
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 congure 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 aribute) 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 rst 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 specied time, its
virtual machine is restarted.
You can congure 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 eective compromise for your needs.
You can also specify custom values for both monitoring sensitivity and the I/O stats interval by selecting the
Custom checkbox.
Table 2‑1. VM Monitoring Settings
Setting Failure Interval (seconds) Reset Period
High 30 1 hour
Medium 60 24 hours
Low 120 7 days
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 15