6.7

A virtual machine "split-brain" condition can occur when a host becomes isolated or partitioned from a
master host and the master host cannot communicate with it using heartbeat datastores. In this situation,
the master host cannot determine that the host is alive and so declares it dead. The master host then
attempts to restart the virtual machines that are running on the isolated or partitioned host. This attempt
succeeds if the virtual machines remain running on the isolated/partitioned host and that host lost access
to the virtual machines' datastores when it became isolated or partitioned. A split-brain condition then
exists because there are two instances of the virtual machine. However, only one instance is able to read
or write the virtual machine's virtual disks. VM Component Protection can be used to prevent this split-
brain condition. When you enable VMCP with the aggressive setting, it monitors the datastore
accessibility of powered-on virtual machines, and shuts down those that lose access to their datastores.
To recover from this situation, ESXi generates a question on the virtual machine that has lost the disk
locks for when the host comes out of isolation and cannot reacquire the disk locks. vSphere HA
automatically answers this question, allowing the virtual machine instance that has lost the disk locks to
power off, leaving just the instance that has the disk locks.
Virtual Machine Dependencies
You can create dependencies between groups of virtual machines. To do so, you must first create the VM
groups in the vSphere Client by going to the Configure tab for the cluster and selecting VM/Host
Groups. Once the groups have been created, you can create restart dependency rules between the
groups by browsing toVM/Host Rules and in the Type drop-down menu, select Virtual Machines to
Virtual Machines. These rules can specify that certain VM groups cannot be restarted until other,
specified VM groups have been Ready first.
Factors Considered for Virtual Machine Restarts
After a failure, the cluster's master host attempts to restart affected virtual machines by identifying a host
that can power them on. When choosing such a host, the master host considers a number of factors.
File accessibility Before a virtual machine can be started, its files must be accessible from
one of the active cluster hosts that the master can communicate with over
the network
Virtual machine and
host compatibility
If there are accessible hosts, the virtual machine must be compatible with
at least one of them. The compatibility set for a virtual machine includes the
effect of any required VM-Host affinity rules. For example, if a rule only
permits a virtual machine to run on two hosts, it is considered for placement
on those two hosts.
Resource reservations Of the hosts that the virtual machine can run on, at least one must have
sufficient unreserved capacity to meet the memory overhead of the virtual
machine and any resource reservations. Four types of reservations are
considered: CPU, Memory, vNIC, and Virtual flash. Also, sufficient network
ports must be available to power on the virtual machine.
vSphere Availability
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