When the master host in a vSphere HA cluster can not communicate with a slave host over the management network, the master host uses datastore heartbeating to determine whether the slave host has failed, is in a network partition, or is network isolated. If the slave host has stopped datastore heartbeating, it is considered to have failed and its virtual machines are restarted elsewhere.

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.

You can use the advanced attribute das.heartbeatdsperhost to change the number of heartbeat datastores selected by vCenter Server for each host. The default is two and the maximum valid value is five.

vSphere HA creates a directory at the root of each datastore that is used for both datastore heartbeating and for persisting the set of protected virtual machines. The name of the directory is .vSphere-HA. Do not delete or modify the files stored in this directory, because this can have an impact on operations. Because more than one cluster might use a datastore, subdirectories for this directory are created for each cluster. Root owns these directories and files and only root can read and write to them. The disk space used by vSphere HA depends on several factors including which VMFS version is in use and the number of hosts that use the datastore for heartbeating. With vmfs3, the maximum usage is approximately 2GB and the typical usage is approximately 3MB. With vmfs5 the maximum and typical usage is approximately 3MB. vSphere HA use of the datastores adds negligible overhead and has no performance impact on other datastore operations.