VMware vSphere 4 Technical Information download pdf (Page 33)

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Chapter 3 Guest Operating Systems

Guest Operating System Networking Considerations

The default virtual network adapter emulated inside a guest is either an AMD PCnet32 device (vlance) or an

Intel E1000 device (E1000). VMware also offers the VMXNET family of paravirtualized network adapters,

however, that provide better performance than these default adapters and should be used for optimal

performance within any guest operating system for which they are available.

The paravirtualized network adapters in the VMXNET family implement an idealized network interface that

passes network traffic between the virtual machine and the physical network interface cards with minimal

overhead. Drivers for VMXNET-family adapters are available for most guest operating systems supported by

ESX.

The VMXNET family contains VMXNET, Enhanced VMXNET (available since ESX 3.5), and VMXNET

Generation 3 (VMXNET3; newly-added in ESX 4.0).

 For the best performance, use the VMXNET3 paravirtualized network adapter for operating systems in

which it is supported. This requires that the virtual machine use virtual hardware version 7, and that

VMware Tools be installed in the guest operating system.

 For guest operating systems in which VMXNET3 is not supported, or if you don’t wish to use virtual

hardware version 7 (to maintain VMotion compatibility with older versions of ESX, for example), the best

performance can be obtained with the use of Enhanced VMXNET for operating systems in which it is

supported. This requires that VMware Tools be installed in the guest operating system.

 For the operating systems in which Enhanced VMXNET is not supported, use the flexible device type. In

ESX, the “flexible NIC” automatically converts each vlance network device to a VMXNET device (a

process also called “NIC Morphing”) if the VMware Tools suite is installed in the guest and the operating

system supports it.

 The VMXNET3 and Enhanced VMXNET devices support jumbo frames for better performance. (Note

that the E1000 and vlance devices do not support jumbo frames.) To enable jumbo frames, set the MTU

size to 9000 both in the guest and in the virtual switch configuration. The physical NICs at both ends and

all the intermediate hops/routers/switches must also support jumbo frames.

 In ESX 4.0, TSO is enabled by default in the VMkernel, but is supported in guests only when they are using

the VMXNET3 device, the Enhanced VMXNET device, or the E1000 device. TSO can improve

performance even if the underlying hardware does not support TSO.

 In some cases, low receive throughput in a virtual machine can be caused by insufficient receive buffers

in the receiver network device. If the receive ring in the guest operating system’s network driver

overflows, packets will be dropped in the VMkernel, degrading network throughput. A possible

workaround is to increase the number of receive buffers, though this may increase the host physical CPU

workload.

OTE The network speeds reported by the guest network driver on the virtual machine do not necessarily

reflect the actual speed of the underlying physical network interface card. For example, the vlance guest driver

on a virtual machine reports a speed of 10Mbps, even if the physical card on the server is 100Mbps or 1Gbps,

because the AMD PCnet cards that ESX emulates are 10Mbps. However, ESX is not limited to 10Mbps and

transfers network packets as fast as the resources on the physical server machine allow

OTE A virtual machine with a VMXNET3 device cannot VMotion to an ESX host running 3.5.x or

earlier.

OTE A virtual machine with an Enhanced VMXNET device cannot VMotion to an ESX host running

3.0.x or earlier.

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