1. The Number Of Vsphere Ha Heartbeat Datastores
2. The Number Of Vsphere Ha Heartbeat Datastores For This Host

I was busy updating my vSphere lab from 4.1 to 5 and ran into a warning on the first ESXi host I updated to ESXi 5.0. It read: “The number of vSphere HA heartbeat datastores for this host is 1, which is less than required: 2”. The message itself is fairly self-explanatory, but prompted. May 8, 2014 Storage vSphere HA If you are configuring a cluster with less than 2 datastores, you will receive an HA warning “The number of heartbeat datastores for host is 1, which is less than required: 2” You can add an option to the HA Advanced Options to supress this warning.

It means you don't have any heartbeat datastores configured.

Here's something on them (with screenshots):

I believe this new Datastore Heartbeat feature is part of making HA more intelligent and able to make better decisions in the case of the Master HA Host being isolated or split off from other hosts. This Datastore Heartbeat feature should help significantly in the case of HA initiated restarts, allowing HA to more accurately determine the difference between a failed host and a host that has just been split off from the others for example.

The Number Of Vsphere Ha Heartbeat Datastores

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The Number Of Vsphere Ha Heartbeat Datastores For This Host

this mechanism works with file locks on the datastores elected for this purpose. HA is able to determine whether the host has failed or is just isolated or split on the network by looking at whether these files have been updated or not.