VMware vSphere Performance – Part Two: Optimizing ESXi Host Networking

An ESXi host system requires a minimum of one network interface card (NIC), for redundancy a host system should be configured with a minimum of at least two NICs in order to satisfy VMkernal and virtual machine demand. For virtual machine communication on a host , these tasks will consume CPU resources and there for will require sufficient CPU resources to also be available for concurrent VMkernal and vMotion network activity.

In some instances (as discussed in http://wp.me/p15Mdc-ua) Direct I/O may be configured to ensure high throughput for a virtual machine workload, which requires the host system to have hardware-assisted virtualization (Intel VT-d or AMD-Vi) enabled in the BIOS.

A host system can use multiple physical CPUs to process network packets from a single network adapter by enabling SplitRx mode, which can improve performance of specific workloads in particular multicast traffic. By default, the feature is automatically enabled on VMXNET3 virtual network adapters , where the host system will detect a single network queue on a physical NIC which is heavily utilised and servicing eight virtual machine network adapters with evenly distributed loads.

SplitRx mode can be enabled on the host system by modifying the ‘Net.NetSplitRxMode’ configuration parameter, by default SplitRx mode is enabled with the value of ‘1’.  To disable SplitRx mode using the v Sphere Web Client select a host system and browse to Manage > Settings > System > Advanced Host System Settings and edit the value of the configuration parameter  ‘Net.NetSplitRxMode’ value to be ‘0’.

Alternatively, we can retrieve the current value of the configuration parameter using the ‘Get-AdvancedSetting’ cmdlet and configure the value using the ‘Set-AdvancedSetting’ cmdlet

Get-AdvancedSetting -Entity esxi1host.domain.local -Name Net.NetSplitRXMode | Select Entity, Name, Value
Get-AdvancedSetting -Entity esxi1host.domain.local -Name Net.NetSplitRXMode | Set-AdvancedSetting -Value 0 -Confirm:$False
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