If you select 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 from the VMware Tools process running inside the guest. If no heartbeats 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.
You can also 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 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.
After failures are detected, VMware HA resets virtual machines. This helps ensure that services remain available. To avoid resetting virtual machines repeatedly for nontransient errors, by default virtual machines will be reset only three times during a certain configurable time interval. After virtual machines have been reset three times, VMware HA makes no further attempts to reset the virtual machines after any subsequent failures until after the specified time has elapsed. You can configure the number of resets using the Maximum per-VM resets custom setting.
Occasionally, virtual machines that are still functioning properly stop sending heartbeats. To avoid unnecessarily resetting such virtual machines, 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 attribute das.iostatsInterval.
Note
The VM Monitoring settings cannot be configured though advanced attributes. Modify settings in the VM Monitoring page of the cluster’s Settings dialog box.
The default settings for VM Monitoring sensitivity are described in the table.
Setting | Failure Interval (seconds) | Reset Period |
|---|---|---|
High | 30 | 1 hour |
Medium | 60 | 24 hours |
Low | 120 | 7 days |
You can specify custom values for both VM Monitoring sensitivity and the I/O stats interval, as described in Customizing VMware HA Behavior.
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Source: http://docphy.com/technology/computers/software/vm-monitoring.html
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