6.5.1

Network Partitions
When a management network failure occurs for a vSphere HA cluster, a subset of the cluster's hosts might
be unable to communicate over the management network with the other hosts. Multiple partitions can occur
in a cluster.
A partitioned cluster leads to degraded virtual machine protection and cluster management functionality.
Correct the partitioned cluster as soon as possible.
n
Virtual machine protection. vCenter Server allows a virtual machine to be powered on, but it can be
protected only if it is running in the same partition as the master host that is responsible for it. The
master host must be communicating with vCenter Server. A master host is responsible for a virtual
machine if it has exclusively locked a system-dened le on the datastore that contains the virtual
machine's conguration le.
n
Cluster management. vCenter Server can communicate with the master host, but only a subset of the
slave hosts. As a result, changes in conguration that aect vSphere HA might not take eect until after
the partition is resolved. This failure could result in one of the partitions operating under the old
conguration, while another uses the new seings.
Datastore Heartbeating
When the master host in a VMware vSphere
®
High Availability cluster cannot communicate with a
subordinate host over the management network, the master host uses datastore heartbeating to determine
whether the subordinate host has failed, is in a network partition, or is network isolated. If the subordinate
host has stopped datastore heartbeating, it is considered to have failed and its virtual machines are restarted
elsewhere.
VMware 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 LUN or NFS server.
You can use the advanced option 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 ve.
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 les 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 les 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 2GB and the typical usage is 3MB. With
vmfs5, the maximum and typical usage is 3MB. vSphere HA use of the datastores adds negligible overhead
and has no performance impact on other datastore operations.
vSphere HA limits the number of virtual machines that can have conguration les on a single datastore.
See Conguration Maximums for updated limits. If you place more than this number of virtual machines on a
datastore and power them on, vSphere HA protects virtual machines only up to the limit.
N A vSAN datastore cannot be used for datastore heartbeating. Therefore, if no other shared storage is
accessible to all hosts in the cluster, there can be no heartbeat datastores in use. However, if you have storage
that is accessible by an alternate network path independent of the vSAN network, you can use it to set up a
heartbeat datastore.
Chapter 2 Creating and Using vSphere HA Clusters
VMware, Inc. 17