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VMWare vSphere 4 offers a new Fault Tolerance system that ...

VMware Fault Tolerance, based on vLockstep technology, provides zero downtime, zero data loss continuous availability for your applications, without the cost and complexity of traditional hardware or software clustering solutions

Is this the answer for the SMB that wants relaible servers? Assuming we have a SAN, two compatible servers, and the cash for the VMWare licenses -- is there any downside to VMware FT? Why wouldn't everyone be running FT?

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How do one provide zero-anything with "a SAN" - doesn't it need to be replicated and properly fail-over switched as well to not be a single-point-of-failure... and isn't that normally pretty expensive? (perhaps not, storage is not my cuppa normally ;) –  Oskar Duveborn Aug 19 '09 at 18:36
    
A good SAN will have redundant network fabrics, redundant controllers on the array(s), redundant HBA's on the server(s) and should come with hot sparing and live volume migration to limit the impact of RAID rebuilds, amongst other things. But yes, that does cost quite a bit. Still it costs less today than it used to and hopefully the trend will continue. –  Helvick Aug 19 '09 at 22:01
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3 Answers

With FT, for every VM that utilizes this feature, you will need one additional VM (obviously) on a secondary ESX host. The overhead is additional CPU and memory usage on the hosts, which will be relative to the workload of VM.

Please see this very detailed blog post on VMware FT: http://communities.vmware.com/blogs/vmroyale/2009/05/18/vmware-fault-tolerance-requirements-and-limitations

It may answer all of your questions.

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Given your scenario the downsides that I see are:

  • Snapshotting is disabled (although Array level snapshotting may get around this)
  • VCB and I presume any 3rd party backups using the vStorage API's can't be used
  • Some performance enhancements in vSphere are not (yet) supported - NPT\EPT, paravirtualized drivers being the two big ones that I can see.
  • You need to guarantee 100% memory availablity for the FT VM's on both primary and secondary plus the overhead (probably another 10-20% on both hosts to be safe)
  • CPU overhead is estimated to be an additional 5-20% above the standard virtualization overhead.
  • The single cpu limit could be a deal breaker.
  • Only a very specific set of newer CPU's are supported - basic hardware virtualization support is not sufficient as far as I can see.
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Isn't it single-vCPU right now? I'd call that a big limitation.

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