VMware 4 Manual

next previous

VMWARE WHITE PAPER

2. Performance Aspects and Best Practice Recommendations

This section describes the performance aspects of Fault Tolerance with best practices recommendations to maximize performance.

For operational best practices please refer to the VMware Fault Tolerance Recommendations and Considerations on VMware

vSphere 4 White Paper.

2.1. FT Operations: Turning On and Enabling

There are two types of FT operations that can be performed on a virtual machine: Turning FT on or o, and enabling or disabling FT.

The performance implications of these operations are as follows:

“Turn On FT” prepares the virtual machine for FT.

• WhenFTisturnedon,devicesthatarenotsupportedwithFTarepromptedforremoval,andthevirtualmachinesmemory

reservation is set to its memory size to prevent ballooning or swapping.

• Useofprocessor’shardwareMMUfeature(AMDRVI/IntelEPT)resultsinnon-determinismandthereforeitisnotsupported

with FT. When FT is turned on for a virtual machine, hardware MMU feature is disabled for that virtual machine. However, virtual

machines that don’t have FT turned on can take advantage of hardware MMU on the same host.

• TurningonFTwillnotsucceedifthevirtualmachineispoweredonandisusingHardwareMMU.Inthiscase,thevirtualmachine

first needs to be either powered off, or migrated to a host that does not have hardware MMU. Similarly turning off FT on a

powered-on virtual machine would not make the virtual machine automatically use hardware MMU; the virtual machine would

need to be powered off and powered back on or migrated to a host that supports hardware MMU for the changes to take effect.

Please see KB article 1008027 for more information on which guest OS and CPU combination requires power on/off operations

for changes to take effect.

“Enable FT” operation enables Fault Tolerance by live-migrating the virtual machine to another host to create a secondary

virtual machine.

• Sincelive-migrationisaresource-intensiveoperation,limitingthefrequencyofenable/disableFToperationsisrecommended.

• Thesecondaryvirtualmachineusesadditionalresourcesonyourcluster.ThereforeiftheclusterhasinsufficientCPUormemory

resources, the secondary will not be created.

When “Turn on FT” operation succeeds for a virtual machine that is already powered on, it automatically creates a new secondary

virtualmachine.Soithasthesameeectas“EnablingFT”.

2.2. Resource Consumption

The additional resource requirements for running a virtual machine with Fault Tolerance enabled are as follows:

• CPUcyclesandmemoryforrunningthesecondaryvirtualmachine

• CPUcyclesforrecordingontheprimaryhostandreplayingonthesecondaryhost

• CPUcyclesforsendingFTloggingtrafficfromtheprimaryhostandreceivingitonthesecondary

• NetworkbandwidthfortheFTloggingtraffic

Record and replay may consume dierent amounts of CPU depending on the event being recorded and replayed, and as a result,

slight dierences in the CPU utilization of the primary and the secondary virtual machines is common and can be ignored.

2.3. Secondary Virtual Machine Execution Speed

As explained in section 1.3, the hypervisor may slow down the primary virtual machine if the secondary is not keeping up pace with

the primary. Secondary virtual machine execution can be slower than the primary for a variety of reasons:

• ThesecondaryhosthasaCPUwithasignificantlylowerclockfrequency

• Powermanagementisenabledonthesecondaryhost,causingtheCPUfrequencytobescaleddown

• ThesecondaryvirtualmachineiscontendingforCPUwithothervirtualmachines