http://www.vmware.com/files/pdf/perf-vsphere-fault_tolerance.pdf - VMware vSphere™ 4 Fault Tolerance: Architecture and Performance
http://www.vmware.com/files/pdf/resources/ft_virtualization_wp.pdf - Protecting Mission-Critical Workloads with VMware Fault Tolerance
http://kb.vmware.com/kb/1010601 - Understanding VMware Fault Tolerance
http://kb.vmware.com/kb/1013428 - VMware Fault Tolerance FAQ
http://www.virtualizationteam.com/virtualization-vmware/vmware-esx-40-ft-fault-tolerant-sneak-peek.h... - VMware ESX 4.0 FT – Fault Tolerant – Sneak Peek
Demo of VMware's vSphere Fault Tolerance (FT)
Only some CPU can be used: http://kb.vmware.com/kb/1008027 - Processors and guest operating systems that support VMware Fault Tolerance
Hardware Virtualization (HV) must be enabled in the BIOS for each host in the cluster (but this is also required for EVC and x64 VMs)
The ESX hosts that run the primary and secondary Fault Tolerance nodes, must be running the same build of ESX.
All ESX hosts used by VMware Fault Tolerance must be members of a VMware High Availability (HA) cluster. VMware HA must be enabled for VMware Fault Tolerance to function.
Networking must be configured properly to ensure proper VMware Fault Tolerance functionality.
In some cases the VM must be powered off to enable FT (for some guest type and to convert vmdk from zeroedthick to eagerzeroedthick )
To check compatibility there is also a VMware utility:
Site Survey - http://www.vmware.com/download/shared_utilities.html
Only 1 vCPU for each VM protected by FT
Require compatible CPU (http://kb.vmware.com/kb/1008027)
No vmxnet3 support (http://kb.vmware.com/kb/1013757)
No snapshot support (and no VCB and other snap-based backup program)
No physical RDM ()
Do not use both ESX and ESXi hosts in clusters with fault tolerant virtual machines (http://kb.vmware.com/kb/1013637)
VM memory will be reserved to the max value
Only eagerzeroedthick disks are supported ()
Seems that Linux guest with VMI are not supported for FT
Use dedicated 1Gb (better if is 10Gbs) link for FT logging
Use SVMotion to converter from thin to thick, or use dastore browser/inflate (but in this case the VM must be powered off)
Use CPU with similar speed
Do not use CPU power management
With Nehalem CPU, HT could be enable without problem (and is also suggested by VMware)
Enable FT with the VM powered off
See also: http://www.vmware.com/files/pdf/fault_tolerance_recommendations_considerations_on_vmw_vsphere4.pdf
FT does not protect against OS or application failures, only from ESX host failures.
Could be useful to implement some backup solution on FT protected VM.