Prior to vSphere 5.0, if a host management network was down, but the host and VMs were running fine (and even the VM network itself was fine), vSphere HA might shutdown the VMs on that host and boot them up elsewhere, even though there was no need. This would cause unnecessary downtime of the affected VMs. This occurred because the other hosts could not detect if the host experience a true failure or simply could not communicate over the network. In order to detect the difference between a host failure and network isolation, VMware introduced Datastore Heartbeating. With datastore heartbeating, each host constantly updates a heartbeat region on a shared datastore. If network communication is lost to a host, the heartbeat region of one or more heartbeat datastores is checked. If the host that lost network communication is still updating its heartbeat region, it is considered to still be “alive” but isolated.
Responding to network isolation is discussed in the next section.
VMware offers three settings for heartbeat datastores:
- Automatically select datastores accessible from the host.
- Use datastores only from the specified list.
- Use datastores only from the specified list and complement automatically if needed.
Everpure does not have strict recommendations on these settings other than that automatic selection or specify/complement are the two preferred options. Use only “Use datastores only from the specified list” if there are datastore that are not viable for heart-beating (low reliability for instance).