This blog entry continues to get a lot of hits, so I thought I would keep it updated and reformat it a bit. VMware’s Fault Tolerance is a great feature that has generated a lot of interest, and it is also a new feature of vSphere that will only continue to improve. With that being said, the list below is the current state of requirements and limitations for enabling FT virtual machines in vSphere. The majority of this information came from the vSphere Pre-requisites Checklist, the VMware Fault Tolerance Datasheet and the Availability Guide. Other items were picked up in the forums or in the VMware knowledge base. kb article 1010601 "Understanding VMware Fault Tolerance" is a great kb resource to start with, if you are new to this feature.

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INFRASTRUCTURE:
VMware FT is available in the following versions of vSphere: Advanced, Enterprise, Enterprise Plus
A host must be certified by the OEM as FT-capable. Refer to the current Hardware Compatibility List (HCL) for a list of FT-supported servers.
Ensure that HV (Hardware Virtualization) is enabled in the BIOS.
Ensure that FT protected virtual machines are on shared storage (FC, iSCSI or NFS).
Ensure that the primary and secondary ESX hosts and virtual machines are in an HA-enabled cluster.
Ensure that there is no requirement to use DRS for VMware FT protected virtual machines; in this release VMware FT cannot be used with VMware DRS (although manual VMotion is allowed).
Ensure that the primary and secondary ESX/ESXi hosts are running the same build of VMware ESX/ESXi. Note: kb 1013637 published September 25, 2009 states that "When creating a cluster that will have fault tolerant virtual machines, the cluster should consist of all ESX hosts or all ESXi hosts and not a mix of ESX and ESXi hosts."
When you upgrade hosts that contain fault tolerant virtual machines, ensure that the Primary and Secondary VMs continue to run on hosts with the same ESX/ESXi version and patch level.
Ensure that there are will be no more than four (to eight) VMware FT enabled virtual machine primaries or secondaries on any single ESX/ESXi host (suggested general guideline based on ESX/ESXi host and VM size and workloads which can vary.)
Ensure that at least gigabit NICs are used. (10 Gbit NICs can be used as well as jumbo frames enabled for better performance.) Each host must have a VMotion and a Fault Tolerance Logging NIC configured. The VMotion and FT logging NICs must be on different subnets.
Ensure that host certificate checking is enabled (enabled by default) before you add the ESX/ESXi host to vCenter Server.
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.
VMware Fault Tolerance requires a dedicated Gigabit Ethernet network between the physical servers, 10 Gigabit Ethernet should be considered if VMware FT is enabled for many virtual machines on the same host.
There are no limits on how many virtual machines in a VMware DRS or VMware HA cluster can be enabled for VMware FT, but every machine with VMware FT enabled takes up twice as much capacity; this should be built into the configuration.
Overhead is dependent on the workload and can be as low as 5% or as much as 20%.
If firewalls or other controls exist between ESX hosts, ports 8100, 8200 (Outgoing TCP, incoming and outgoing UDP) must be open.
Ensure that a resource pool containing fault tolerant virtual machines has excess memory above the memory size of the virtual machines. Fault tolerant virtual machines use their full memory reservation. Without this excess in the resource pool, there might not be any memory available to use as overhead memory.
To ensure redundancy and maximum Fault Tolerance protection, VMware recommends that you have a minimum of three hosts in the cluster. In a failover situation, this provides a host that can accommodate the new Secondary VM that is created.
Too Much Activity on VMFS Volume Can Lead to Virtual Machine Failovers – reduce the number of file system operations or ensure that the fault tolerant virtual machine is on a VMFS volume that does not have an abundance of other virtual machines that are regularly being powered on, powered off, or migrated using VMotion.
When Fault Tolerance is turned on, vCenter Server unsets the virtual machine’s memory limit and sets the memory reservation to the memory size of the virtual machine. While Fault Tolerance remains turned on, you cannot change the memory reservation, size, limit, or shares.
Disabling the virtual machine restart priority setting for a fault tolerant virtual machine causes the Turn Off Fault Tolerance operation to fail. In addition, fault tolerant virtual machines with the virtual machine restart priority setting disabled cannot be deleted.
FT requires that the hosts for the Primary and Secondary VMs use the same CPU model, family, and stepping.
Hosts running the Primary and Secondary VMs should operate at approximately the same processor frequencies, otherwise the Secondary VM might be restarted more frequently. Platform power management features which do not adjust based on workload (for example, power capping and enforced low frequency modes to save power) can cause processor frequencies to vary greatly.
VMware Consolidated Backup (VCB) is not supported with Fault Tolerance – A VCB backup performed on an FT-enabled virtual machine powers off both the primary and the secondary virtual machines and might render the virtual machines unusable. A recent discussion over in the forums hints that VCB support may be on the way!
Apply the same instruction set extension configuration (enabled or disabled) to all hosts.
Ensure that the processors are supported: AMD Barcelona+, Intel Penryn+ (Download VMware SiteSurvey.) For VMware FT to be supported, the servers that host the virtual machines must each use a supported processor from the same category as documented below:
Intel Xeon based on 45nm Core 2 Microarchitecture Category:

• 3100 Series
• 3300 Series
• 5200 Series (DP)
• 5400 Series
• 7400 Series

Intel Xeon based on Core i7 Microarchitecture Category:

• 3500 Series
• 5500 Series

AMD 3rd Generation Opteron Category:

• 1300 and 1400 Series
• 2300 and 2400 Series (DP)
• 8300 and 8400 Series (MP) – kb 1008027

VIRTUAL MACHINES:
Virtual machines must be running on one of the supported guest operating systems. See VMware kb 1008027 for more information.
The combination of the virtual machine’s guest operating system and processor must be supported by Fault Tolerance (for example, 32-bit Solaris on AMD-based processors is not currently supported).
VMware FT requires virtual machines to have thick-eager zeroed disks. Thin or sparsely allocated disks will be converted to thick-eager zeroed when VMware FT is enabled requiring additional storage space. The virtual machine must be in a powered-off state to take this action.
Ensure that the datastore is not using physical RDM (Raw Disk Mapping). Virtual RDM is supported.
VMware recommends that you use a maximum of 16 virtual disks per fault tolerant virtual machine.
Ensure that there is no requirement to use Storage VMotion for VMware FT VMs, since Storage VMotion is not supported for VMware FT VMs.
Ensure that NPIV (N-Port ID Virtualization) is not used, since NPIV is not supported with VMware FT.
Ensure that the virtual machines are NOT using more than 1 vCPU. (SMP is not supported.)
Ensure that there is no user requirement to hot add or remove devices since hot plugging devices cannot be done with VMware FT.
Ensure that there is no user requirement to use USB (USB must be disabled) and sound devices (must not be configured) since these are not supported for Record/Replay (and VMware FT.)
Ensure that there is no user requirement to have virtual machine snapshots since these are not supported for VMware FT. Delete snapshots from existing virtual machines before protecting with VMware FT. Note: Client agents may be required for backups.
Ensure that virtual machine hardware is upgraded to v7.
Ensure that the virtual machines do not use a paravirtualized guest OS. Note: On September 22, 2009 it was announced that support for guest OS paravirtualization using VMware VMI to be retired from new products in 2010-2011.
Fault Tolerance is not supported with Paravirtual SCSI adapters.
The vmxnet3 adapter is not supported with Fault Tolerance. See kb 1013757. Also posted in comments by {user:username=andreleibovici} – thanks!*
Some legacy network drivers are not supported. vmxnet2 is, but you might need to install VMware tools to access the vmxnet2 driver instead of vlance in certain guest operating systems.
Ensure MSCS clustered virtual machines will have MSCS clustering removed prior to protecting with VMware FT.
VMs can’t have any non-replayable devices (USB, sounds, physical CD-ROM, physical floppy)
The virtual machine must not be a template.
The virtual machine must not have VMware HA disabled.
VMDirectPath is not available for FT virtual machines.
VMCI stream socket connections are dropped when a virtual machine is put into Fault Tolerance (FT) mode. No new VMCI stream socket connections can be established while in FT mode.
The hot plug device feature is automatically disabled for fault tolerant virtual machines. To hot plug devices, you must momentarily turn off Fault Tolerance, perform the hot plug, and then turn on Fault Tolerance.
Extended Page Tables (EPT)/Rapid Virtualization Indexing (RVI) is automatically disabled for virtual machines with Fault Tolerance turned on.
Software virtualization with FT is unsupported.

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