In case you are already running ESXi 6.0, be aware of VMware KB2145106. It describes an issue where ESXi 6.0 gets disconnected and appears as Not Responding in vCenter. Currently, there is no resolution, apart from disabling the SMART deamon (smartd).
Disabling smartd will affect this data capture and is not generally recommended by VMware, but it is the only workaround provided currently in KB2145106.
To disable smartd daemon:
chkconfig smartd off
The likelihood and impact of this problem are not too high, so we would not advise disabling smartd on all of your ESXi 6.0 hosts, unless you are experiencing the problem already.
If you use Runecast Analyzer for log analysis, you will be alerted if you are experiencing the described issue. You will see at least one Log KB detected on the main dashboard:
Navigate to Discovered KBs view to see details of the detected problems in your logs and their corresponding Knowledge Base articles:
In general, the ESXi host Not Available issue can have many causes, so Runecast can save you some serious troubleshooting time.