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. 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.