A: VMware introduced the Fault Tolerant (FT) feature with vSphere 4. It lets you run multiple instances of a virtual machine (VM) on different hosts. They're kept synchronized through lock-step processor instruction processing, which allows no loss of VM state or availability in the event of an unplanned outage.
This feature requires redundant network connections that are high speed and low latency. This ensures the copies of the VM don't affect the primary VM's performance because of the synchronous nature of the replication.
In vSphere 4, the FT-enabled VM could have only one vCPU (because of the complexities of the FT lock-step process) as documented at VMware's website. In vSphere 5, this maximum has remained as configured in this vSphere PDF.
In the vSphere PDF, Table 2 shows the maximum virtual CPUs for a FT VM. The maximum memory is 64GB, and up to four VMs per host can be FT enabled, which is the same as vSphere 4.1.
To see more FAQs, please go to John Savill's FAQs page on Windows IT Pro.