vSphere Fault Tolerance (FT) explained

vSphere Fault Tolerance (FT) provides a higher level of business continuity than vSphere HA. It works by creating a duplicate (secondary) copy of the virtual machine on a different host and keeping the two VMs in sync. The secondary VM can immediately take over in the event of an ESXi host failure and the entire state of the virtual machine will be preserved.

Because FT provides zero downtime and zero data loss, it is usually used for business-critical applications that must be available all the time. It is also sometimes used for applications that have no native capability for clustering.

vSphere FT also has some disadvantages. Here are the main ones:

• increased resource usage. An FT-protected VM will use twice as much resources. For example, if the primary VM uses 2GB of RAM, the secondary VM will also use 2GB of RAM.
• only virtual machines with a single vCPU are compatible with Fault Tolerance.
• hosts must be licensed for vSphere FT.
• the VM must not have any snapshots.