You might want to reference the documentation from Roger
Cluster Prerequisites
Unlike VMware HA which, by default, protects every virtual machine in the cluster, VMware Fault Tolerance is enabled on individual virtual machines. For a cluster to support VMware Fault Tolerance, the following prerequisites must be met:
■ VMware HA must be enabled on the cluster. Host Monitoring should also be enabled. If it is not, when Fault Tolerance uses a Secondary VM to replace a Primary VM no new Secondary VM is created and redundancy is not restored.
■ Host certificate checking must be enabled for all hosts that will be used for Fault Tolerance. See Enable Host Certificate Checking.
■ Each host must have a VMotion and a Fault Tolerance Logging NIC configured. See Configure Networking for Host Machines.
■ At least two hosts must have processors from the same compatible processor group. While Fault Tolerance supports heterogeneous clusters (a mix of processor groups), you get the maximum flexibility if all hosts are compatible. See the VMware knowledge base article at http://kb.vmware.com/kb/1008027 for information on supported processors.
■ All hosts must have the same ESX/ESXi version and patch level.
■ All hosts must have access to the virtual machines' datastores and networks.”
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Host Prerequisites
A host can support fault tolerant virtual machines if it meets the following requirements.
■ A host must have processors from the FT-compatible processor group. See the VMware knowledge base article at http://kb.vmware.com/kb/1008027.
■ A host must be certified by the OEM as FT-capable. Refer to the current Hardware Compatibility List (HCL) for a list of FT-supported servers (see http://www.vmware.com/resources/compatibility/search.php).
■ The host configuration must have Hardware Virtualization (HV) enabled in the BIOS. Some hardware manufacturers ship their products with HV disabled. The process for enabling HV varies among BIOSes. See the documentation for your hosts' BIOSes for details on how to enable HV. If HV is not enabled, attempts to power on a fault tolerant virtual machine produce an error and the virtual machine does not power on.
Before Fault Tolerance can be turned on, a virtual machine must meet minimum requirements.
■ Virtual machine files must be stored on shared storage. Acceptable shared storage solutions include Fibre Channel, (hardware and software) iSCSI, NFS, and NAS.
■ Virtual machines must be stored in virtual RDM or virtual machine disk (VMDK) files that are thick provisioned with the Cluster Features option. If a virtual machine is stored in a VMDK file that is thin provisioned or thick provisioned without clustering features enabled and an attempt is made to enable Fault Tolerance, a message appears indicating that the VMDK file must be converted. Users can accept this automatic conversion (which requires the virtual machine to be powered off), allowing the disk to be converted and the virtual machine to be protected with Fault Tolerance. The amount of time needed for this conversion process can vary depending on the size of the disk and the host's processor type.
■ Virtual machines must be running on one of the supported guest operating systems. See the VMware knowledge base article at http://kb.vmware.com/kb/1008027 for more information.”
See more detials at
http://itblog.rogerlund.net/2009/09/vmware-fault-tolerance-what-is-it-what.html