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VMware Fault Tolerance, single vCPU workloads, and performance on modern hardware

Two recent posts on VMware's VROOM! Blog, written by Todd Muirhead (@virtualTodd) in our performance team. The upshot? FT can currently be used only on VM's with one vCPU. With current hardware, however, this may not be as much of a limitation as you think! In the most recent post, Todd walks us through a case where a Nehalem single vCPU VM is doing just as much as a double processor VM running on just slightly older parts.

Comparing Performance of 1vCPU Nehalem VM with 2vCPU Harpertown VM


There are a couple of interesting things to note about the results. 

The first is that the sendmail average latency results with FT enabled on a 1vCPU Xeon 5570 based VM with 1500 users was within 5ms of the 2vCPU Xeon 5460 VM with 2000 users.  This means that the Nehalem based 1vCPU VM was getting an extra 50% more users per vCPU than the 2vCPU Harpertown based VM.

Average CPU utilization on the 1vCPU VM with 2000 users and FT enabled was only 45% which leaves head room for spikes in usage.  This means that 2000 heavy online LoadGen users ran comfortably in a 1vCPU VM. 


A 1vCPU Xeon X5500 series based Exchange Server VM can support 50% more users per core than a 2vCPU VM based on previous generation processors while maintaining the same level of performance in terms of Sendmail latency.  This is accomplished while the VM’s CPU utilization remains below 50%, allowing plenty of capacity for peaks in workload and making an FT VM practical for use with Exchange Server 2007.

In his previous post, Todd looks at the performance impact of FT on the Microsoft Exchange workloads.

Performance of Exchange Server 2007 in a Fault Tolerant Virtual Machine


The testing showed that the performance of the Exchange VM was affected only slightly when FT was used. Sendmail average latency increased by 10 to 13 milliseconds, and 95th percentile avgerage latency increased by 33 to 45 milliseconds.  All test results were under the 1000ms threshold at which user experience starts to degrade.  These results indicate that, even at 2000 users, the performance of Exchange on a 1 vCPU VM was acceptable with or without FT.




The CPU utilization results for the overall system show a low impact of using FT.  Because the Exchange VM was the only one on the ESX server, overall system utilization was very low with a peak of just over 7% in the most stressful test.  Enabling FT only caused an additional 1 to 1.5% of system CPU to be used.  The utilization of the ESX host with the secondary VM was slightly lower than the primary.  When examining the CPU utilization of the 1 vCPU VM, the utilization average reaches just under 45%.  This is a comfortable level that still leaves room for the bursty nature of Exchange. 

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