Q. What is VMware Fault Tolerance (FT)?

A. vSphere introduces FT to the virtualization platform, including a zero loss fault tolerance solution. With normal high availability such as with vMotion and Hyper-V's Live Migration, there's a zero-downtime migration solution in planned scenarios. For example, you can select to move a virtual machine (VM) from one host to another. In the event of a host crash, technologies such as vMotion and Live Migration can't be used, because there's no time to copy the memory from the running VM.

FT in VMware ESX 4 (vSphere) is a zero-loss fault tolerant solution that actually runs the VM on two ESX hosts at the same time, one primary and one secondary, and keeps them in sync through VMware vLockstep technology. This technology makes sure that both VMs have exactly the same state and memory content. If the current active VM host fails, the other VM automatically takes over, with no downtime.

vLockstep captures all nondeterministic events, such as network events, user input, and timer events, on the primary VM's CPU. It sends these events to the second VM over the network, where they're replayed. This means you need a good network, such as redundant gigabit links, for the logging traffic. Because both VMs get the same requests and actions, the resulting states in each of them are identical. Any disk write activity by the second VM is suppressed by the hypervisor, because you don't want disks being written to twice!

There's a list on VMware's site at that you can use to check if FT supports your CPUs. The site also offers a good whitepaper on FT.

Related Reading:

Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at ITTV.net.