By default, vSphere Fault Tolerance can accommodate SMP virtual machines with up to four vCPUs. If your virtual machine has only a single vCPU, however, you can use legacy FT instead for backward compatibility. Unless legacy FT is technically necessary, avoid using it.

To use legacy Fault Tolerance, you must configure an advanced option for the virtual machine. After you complete this configuration, the legacy FT VM is different in some ways from other vSphere FT VMs.

Differences for VMs That Use Legacy FT

VMs that use vSphere FT and VMs that use legacy FT differ in several ways.

Table 1. Differences Between Legacy FT and vSphere FT

Legacy FT

vSphere FT

Extended Page Tables/Rapid Virtualization Indexing (EPT/RVI)

Not supported



Not supported for legacy FT logging NICs.

Supported for vSphere FT-logging NICs.


Fully supported for initial placement, load balancing, and maintenance mode support.

Only power on placement of Secondary VM and maintenance mode are supported.

vStorage APIs - Data Protection backups

Not supported


Eager-zeroed thick .vmdk disk files


Not required because vSphere FT supports all disk file types, including thick and thin

.vmdk redundancy

Only a single copy

Primary VMs and Secondary VMs always maintain independent copies, which can be placed on different datastores to increase redundancy.

NIC bandwidth

Dedicated 1-Gb NIC recommended

Dedicated 10-Gb NIC recommended

CPU and host compatibility

Requires identical CPU model and family and nearly identical versions of vSphere on hosts.

CPUs must be compatible with vSphere vMotion or EVC. Versions of vSphere on hosts must be compatible with vSphere vMotion.

Turn on FT on running VM

Not always supported. You might need to power off VM first.


Storage vMotion

Supported only on powered-off VMs.

vCenter Server automatically turns off FT before performing a Storage vMotion action and turns on FT again after the Storage vMotion action completes.

Not supported. User must turn off vSphere FT for the VM before performing the Storage vMotion action and turn on vSphere FT again.

vlance networking drivers

Not supported


Additional Requirements for Legacy FT

In addition to the differences listed for legacy FT, it also has the following unique requirements.

  • ESXi hosts must have access to the same virtual machine datastores and networks.

  • Virtual machines must be stored in virtual RDM or virtual machine disk (VMDK) files that are thick provisioned. If a virtual machine is stored in a VMDK file that is thin provisioned and an attempt is made to use fault tolerance, a message appears. It indicates that the VMDK file must be converted. To perform the conversion, you must power off the virtual machine.

  • Hosts must have processors from the vSphere FT-compatible processor group. Verify that the hosts' processors are compatible with one another.

  • The host that supports the Secondary VM must have a processor that supports fault tolerance and is the same CPU family or model as the host that supports the Primary VM.

  • When upgrading hosts that contain fault tolerant VMs, verify that the Primary and Secondary VMs continue to run on hosts with the same FT version number or host build number. This requirement applies to hosts before ESX/ESXi 4.1.


    If you designated a VM to use legacy FT before you upgraded the hosts in the cluster, that VM continues to use legacy FT after the host upgrade.

Upgrading Hosts Involved with Legacy FT

To upgrade your hosts to vSphere 6.5 or later, you must turn off legacy FT on all affected VMs or move those VMs to other hosts. If you do not prepare for the upgrade in this way, VMware vSphere Update Manager blocks the upgrade.

vCenter Server version 6.5 or later can manage existing legacy FT VMs, but you cannot create legacy FT VMs, even on hosts with a version earlier than version 6.5. The following vSphere FT operations can be performed in this scenario:

  • Suspend or resume FT

  • Test failover

  • Restart secondary

  • Migrate secondary

  • Turn off FT


Legacy FT VMs can exist only on ESXi hosts that are running on vSphere versions earlier than 6.5.