Watch out for a gotcha when using the VMXNET3 virtual adapter

Virtualization expert Rick Vanover shows how using the VMXNET3 virtual network adapter may cause confusion for Windows administrators.

Windows Task Manager is a go-to performance visualization and troubleshooting tool. It does a good job of visualizing CPU, memory, running applications and process information. The tool also provides a quick view of network utilization on all interfaces on a Windows system.

The VMXNET3 is a high performance network interface that is available with vSphere 4 virtual machines. For virtual machines that have the VMXNET3 network interface installed, one feature the interface offers is that the guest operating system will display the media being connected at 10 Gb/s. This is the case even if the physical network media is only 1 Gb/s or slower.

On the Networking tab of Windows Task Manager, the current percentage of network utilization is displayed for each interface. In the case of the VMXNET3 interface, the operating system perceives that this interface has a 10 Gb/s line rate, and its percentages are displayed accordingly. Figure A shows one virtual machine running the VMXNET3 interface with nominal usage. Figure A

Click the image to enlarge.

The percentage of network utilization for this server is nothing to be concerned about -- it's just a few percent utilization. If the utilization were to approach the 10% mark, it would be 100% of the line rate on this underlying ESXi host server. (In this environment, the line rate is 1 Gb/s.)

A number of situations cause the rate to exceed the 10% mark on a 1 Gb/s network. The most obvious is when the transfer is made from one virtual machine to another virtual machine where both are on the same port group and on the same host. In this situation (assuming there is no firewall or advanced network tools in place), the network traffic will process via the CPU of the ESXi server and not traverse the physical networking media.

From a simple visual indicator, the Windows Task Manager tab can offer useful information very quickly. In the midst of troubleshooting, a 1 Gb/s physical network may be saturated if the utilization displays 10% or so due to the perceived faster rate of the VMXNET3 interface.

Have you adapted any troubleshooting skills to the VMXNET3 interface? Let us know in the discussion.