Today, FT has a LOT of caveats. Aside from the obvious one everyone’s talking about (the limitation to 1 CPU VMs), there’s some other ones that I think are much worse (especially if you have newer CPUs) because they don’t just affect one VM, but the whole host. Here’s a few examples:
- Power Management must be turned off in the BIOS on the ESX Host. This is a big bummer, considering ESX4 just started supporting low-power state CPU features like SpeedStep.
- Hyper-Threading must be turned off on the host. If you’ve got a new Xeon 5500 processor, this is a bummer as well.
- Turning on FT disabled EPT/RVI for the ENTIRE host. This means one of the biggest performance enhancements (vMMU support) is gone for your whole host when you turn on FT for a single VM! UPDATE: This only disables FT for the single VM, not all VMs.
And as for VM-specific limitations (meaning these at least only affect the FT VM):
- No Virtual SMP
- No Snapshots (meaning no Storage VMotion, no VCB)
- No Hot-Add hardware
- No NPIV
- No DRS
- No Thin Provisioning
I think FT is a really cool piece of engineering, but today it’s pretty obvious that’s a version 1.0 (or worse, version 0.9) product. It works, but there are more gotchas than in any other VMware feature I’ve ever seen.