VMware vSphere FT is a way to improve the availability level for critical VMs, with a zero-downtime technology.
vSphere FT works by continuously replicating the state of the VM between two different ESXi hosts. As a result there are two identical copies of a VM—the primary VM and the secondary VM (sometimes called shadow VM). Each VM has its own set of configuration files, VMX and VMDK files, which vSphere FT automatically keeps synchronized.
When the physical ESXi server where the primary VM is running fails, the secondary VM (shadow VM) automatically takes over and resumes normal operations.
VMware vSphere FT also has some limits—for each VM, it supports a maximum of 4 vCPUs and 64 GB RAM. For each host, it supports a maximum of 4 fault-tolerant VMs. VMware vMotion migration is supported for both VMs, as are the different virtual disk formats...