I recently did some exploring on my home lab around datastore heatbeats and came up with the following notes around how to determine which ones are active, how to change the defaults, and why vCenter Server might now choose a datastore.
vCenter Server selects a preferred set of datastores for heartbeating. This selection is made to maximize the
number of hosts that have access to a heartbeating datastore and minimize the likelihood that the datastores
are backed by the same storage array or NFS server. To replace a selected datastore, use the Cluster Settings
dialog box of the vSphere Client to specify the heartbeating datastores. The Datastore Heartbeating tab lets
you specify alternative datastores. Only datastores mounted by at least two hosts are available. You can also
see which datastores vSphere HA has selected for use by viewing the Heartbeat Datastores tab of the HA
Cluster Status dialog box.
Only use these settings if you want to override the default vCenter Server Choice
Here is an article around why it might not choose a Datastore…
User-Preferred Datastore is Not Chosen
vCenter Server might not choose a datastore that you specify as a preference for vSphere HA storage heartbeating.
You can specify the datastores preferred for storage heartbeating, and based on this preference, vCenter Server determines the final set of datastores to use. However, vCenter Server might not choose the datastores that you specify.
This problem can occur in the following cases:
■ The specified number of datastores is more than is required. vCenter Server chooses the optimal number of required datastores out of the stated user preference and ignores the rest.
■ A specified datastore is not optimal for host accessibility and storage backing redundancy. More specifically, the datastore might not be chosen if it is accessible to only a small set of hosts in the cluster. A datastore also might not be chosen if it is on the same LUN or the same NFS server as datastores that vCenter Server has already chosen.
■ A specified datastore is inaccessible because of storage failures, for example, storage array all paths down or permanent device loss.
■ If the cluster contains a network partition, or if a host is unreachable or isolated, the host continues to use the existing heartbeat datastores even if the user preferences change.
Verify that all the hosts in the cluster are reachable and have the vSphere HA agent running.
Also, ensure that the specified datastores are accessible to most, if not all, hosts in the cluster and that the datastores are on different LUNs or NFS servers.