VMware 4 Manual

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VMWARE WHITE PAPER

To ensure that the secondary virtual machine runs as fast as the primary, it is recommended that:

• ThehostsintheFTclusterarehomogenous,withsimilarCPUmake,model,andfrequency.TheCPUfrequencydifferenceshould

not exceed 400 MHz.

• Boththeprimaryandsecondaryhostsusethesamepowermanagementpolicy.

• CPUreservationissettofullforcaseswherethesecondaryhostcouldbeoverloaded.TheCPUreservationsettingontheprimary

applies to the secondary as well, so setting full CPU reservation ensures that the secondary gets CPU cycles even when there is

CPU contention.

2.4. I/O Latencies

All incoming network packets to the primary, and all disk reads at the primary, are immediately sent to the secondary. However, as

explained in section 1.4, network transmits and disk writes at the primary are held until the secondary acknowledges that all events

that precede the packet transmit or disk write. As a result, the round-trip network latency between the primary and the secondary

aectstheI/Olatencyofdiskwritesandnetworktransmitoperations.SincetheroundtriplatencyinaLANenvironmentisusuallyin

the order of a few hundred microseconds, and disk I/O latencies are usually on the order of a few milliseconds, this delay does

not impact disk write operations. One may, however, notice delays in network ping responses if the response time is shown in

microseconds. For best performance, it is recommended that the round-trip network latency between the primary and secondary

host be less than 1 millisecond.

2.5. Network Link

Since the primary and secondary virtual machines proceed in vLockstep, the network link between the primary and the

secondary host plays an important role in performance. A Gigabit link is required to avoid congestion. In addition, higher bandwidth

network interfaces generally have lower transmission latency. If the network is congested and the primary host is not able to send

trac to the secondary (i.e. when the TCP window is full), then the primary virtual machine will make little or no forward progress. If

the network connection between the primary and secondary hosts goes down, either the current primary or the current secondary

virtual machine will take over, and the other virtual machine will die.

2.6. NIC Assignments for Logging Traffic

FT generates two types of network trac:

• Migrationtraffictocreatethesecondaryvirtualmachine

• FTloggingtraffic

MigrationtrachappensovertheNICdesignatedforVMotionanditcausesnetworkbandwidthusagetospikeforashorttime.

SeparateanddedicatedNICsarerecommendedforFTloggingtracandVMotiontrac,especiallywhenmultipleFTvirtual

machinesresideonthesamehost.SharingthesameNICforbothFTloggingandVMotioncanaecttheperformanceofFTvirtual

machines whenever a secondary is created for another FT pair or a VMotion operation is performed for any other reason.

VMwarevSwitchnetworkingallowsyoutosendVMotionandFTtractoseparateNICswhilealsousingthemasredundantlinksfor

NICfailover.SeeKB article 1011966 for more information.

Adding multiple uplinks to the virtual switch does not automatically result in distribution of FT logging trac. If there are multiple FT

pairs, then trac could be distributed with IP-hash based load balancing policy, and by spreading the secondary virtual machines to

dierent hosts.

2.7. Virtual Machine Placement

FT logging trac is asymmetric: the bulk of the trac ow happens from the primary to the secondary hosts and the secondary host

only sends back acknowledgements. If multiple primary virtual machines are co-located on the same host, they could all compete for

thesamenetworkbandwidthontheloggingNIC.Idlevirtualmachinesconsumelessbandwidth,butI/O-intensivevirtualmachines

can consume a lot of network bandwidth. It can be helpful to place the primary of one FT pair and the secondary of another FT pair

onthesamehosttobalancethetracontheFTloggingNIC.