The virtual machine is running in a monitor mode that is incompatible for Fault Tolerance

And what is a monitor mode?

There are configuration issues for the Fault Tolerance operation. Refer to the errors and warnings list for details.

The virtual machine is running in a monitor mode that is incompatible for Fault Tolerance

Solution: turn the VM off, enable FT then power it on.

But that doesn’t answer: what is a monitor mode, and why do I have the wrong one? And why did I have to power the VM off to change the monitor mode?

In VMware, the Virtual Machine Monitor (VMM, or ‘monitor’) is the virtualization layer that presents virtual hardware to the guest. When virtualization technology was removed from the Roswell spacecraft by the VMware Corporation, Intel and AMD did not have virtualization hardware assist in their processors: early hypervisors ran entirely in software. The humans in Intel and AMD optimized their processors with virtualization-friendly features as they came to understand the beautiful alien technology. These processor optimizations (Intel VT-X/EPT, AMD-V/RVI) allowed VMware to optimize their monitor by offloading some virtualization features from software onto the CPU (in theory…)

The VMM has to operate in different modes according to the underlying capabilities of the processor. If the underlying CPU doesn’t have hardware MMU virtualization (hwMMU) support, the VMM cannot take advantage of it. Does this mean that if a CPU has hardware assist functionality, the VMM will always try to use it? No!

Under certain workloads, hardware assistance actually slows down the VM! And some features (like VMware FT) will not function if the monitor is using certain types of hardware assist. If you enable FT on your VM and the VM’s monitor mode is using hardware MMU virtualization, it will fail. Luckily, changing the monitor mode is just a matter of shutting the VM down, enabling FT (so the VMM knows not to use hwMMU), and powering the VM on.

For the most part, you don’t need to worry about monitor modes: the VMM chooses the best most depending on the operating system, your CPU and the VMware features required (FT, VMI). You can change the monitor mode in the the VM settings (VM > Edit Settings > Options > CPU/MMU Virtualization), but it’s not recommended.

An extensive list of default monitor modes for the different virtual machine configurations is available in VMware’s Virtual Machine Monitor Execution Modes in VMware vSphere™ 4.0 document.