Practical lessons - Fault Tolerant VM

I have read quite a few official docs and different bloggers' posts about VMware FT, but once again I got evidence that practical knowledge is a best way of studying.

FT seems to be amazing feature, but the long list of requirements and restrictions can stop most of the VMware enthusiasts. Nevertheless, FT is one of the topic of VCAP-DCA blueprint, so I spent half a day playing in my lab with FT and trying to enable FT just for one VM. 

I wrote down some important lessons I learned about it, even though they seem very obvious you need to have some practical experience to remember them forever :)

• The identical CPU model are required - in my lab it didn't work on hosts that had Xeon 5550 and Xeon 5650
• The identical patch/firmware level is required - the same here, hosts had different firmware versions
• The VM Monitor mode has to be set to Use Intel VT-x/AMD-v for CPU virtualization and software for MMU Virtualization". - Yes, I have read that EPT and Large Pages are not supported for FT, but I didn't know it had to be switched off manually.
• VM disk can be converted to thick-eager-zeroed one only when VM is shutdown. - I was quite sure it was doable while VM was running
•  Memory reservation is set to the amount of configured memory. - You need to be very careful enabling FT if you use HA with HA Admission Control configured with hosts number to tolerate. This can impact the slot size of your HA cluster. 
• It is possible to configure Hyperthreading Core Sharing mode to None to provide slightly better performance for FT VM. - Actually, I am not really convinced that it provides better performance for FT enabled VM, but at least it will guarantee no other vCPU will be scheduled with FT VM vCPUs on the same physical CPU. My main idea was to make sure it is compatible with FT. 
• When you disable FT the secondary VM still stays in vSphere, but it powered off and disabled. All historical data about FT performance is kept.
• When you turn off FT the secondary VM is removed and all historical data is delel.
• RDM disks are not supported in FT VM.

I have also found some minor bug when you turn off FT. When you enable FT for VM it warns you about memory reservation and says it reservation will be maintained while FT is on. 

However, when I turned off FT the VM still kept memory reservation. Not a big deal, but it is just good to know you need to check VM settings after FT is switched off. 

There are a lot of other problems/issues you can face with when enabling FT, but my story was not that long :)

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