VM Junkie

June 11, 2009

Why you may want to leave FT alone for now…

Filed under: Uncategorized, vmware, vSphere — ermac318 @ 9:37 am

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.



  1. Actually, it rivals Directpath I/O which has a list about as long. Seriously though, when vMotion was released did every customer run out and do it to all of their production VM’s the first day they installed it? Probably not (unless they have no fear). Customers will kick the tires on it for some time, in the meantime, the limitations should diminish over time.


    Comment by The VMguy — June 11, 2009 @ 12:55 pm

  2. >>Turning on FT disabled EPT/RVI for the ENTIRE host.

    This is incorrect. Turning on FT only disables the use of NPT/EPT for that virtual machine (since the use of NPT/EPT results in non-determinism that is not trackable). Other virtual machines on the host can use NPT/EPT.

    Comment by Krishna — June 11, 2009 @ 3:38 pm

  3. Krishna:
    You are correct. In previous versions of the vSphere migration checklist, the follow item was listed under the Fault Tolerance section:
    “Ensure that there is no user requirement to use NPT/EPT (Nested Page Tables/Extended Page Tables) since VMware FT disables NPT/EPT on the ESX host”
    However, they have since updated it (on June 8th) and it now reads:
    “Ensure that there is no user requirement to use NPT/EPT (Nested Page Tables/Extended Page Tables) on VMware FT protected VMs, since VMware FT disables NPT/EPT for the VMs
    Thanks for the correction! I’m glad I was wrong.

    Comment by ermac318 — June 11, 2009 @ 7:03 pm

  4. For the record, HT does not need to be disabled for FT; the only currently supported CPU with HT is the Xeon 5500, and that CPU will work fine with HT enabled.

    Comment by Eric — June 12, 2009 @ 5:42 am

  5. Eric,
    Again, in the version of the migration checklist from before June 8th, this was listed as a “required” option. It is now listed as “Optional” due to “performance implications”
    So it looks like this shows two things. One: FT may not be as bad as I first thought, but Two: VMware really needs to get their documentation straight!

    Comment by ermac318 — June 12, 2009 @ 8:00 am

  6. […] Other bloggers talking about this issue: Scottlowe.org, VMJunkie.wordpress.com, […]

    Pingback by The list of things FT doesn’t like is … very long! « DeinosCloud — July 10, 2009 @ 1:38 am

  7. I really appreacite the fact that you’ve taken the time to go so “in-depth” regarding this very impo

    Comment by Dave Kellis — September 7, 2009 @ 2:22 pm

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