Determining the cause of an ESXI host power failure / restart

Firstly ensure that there are no warning / error lights on the physical host.

Check the event log for the specific ESXI host by going to;

Host >> Tasks and Events >> Tasks

We should then proceed by enabling SSH from the vSphere Client:

Host >> Configuration >> Security Profile >> Services >> Properties and enable SSH.

SSH into the host and run:

cat /var/log/vmksummary.log

You should typically see a regular heart-beat message - although around the time in question we encountered the folloeing event:

2013-01-01T12:30:04Z bootstop: Host has booted

To determine if it was a deliberate reboot we should check for the following line:

localhost vmkhalt: (1268148282) Rebooting system...

* This line would indicate that the boot was initiated by a user.

We should also be able to tell whether it was initiated by a user from the vCenter logs accessable via the vSphere client:

vCenter (Root node) >> Tasks and Events.

Since this line was absent from the vmksummary.log log file it appears that there might have been a power failure at this point.