Poor Network Performance on Hyper-V VMs in Windows Server 2019

Several times I came across a situation when files were copied much slower from/to virtual machines on a Hyper-V host running Windows Server 2019 than in a VM of the same configuration on a host running Windows Server 2016. In some tests, the read/write speed over the network to VM on Windows Server 2019 is almost three times lower than that on WS2016 (copying over SMB, SSH/SCP was tested). In this article, I tried to describe several different methods to improve the network performance of Hyper-V virtual machines running on Windows Server 2019 (and the latest Windows 10 and 11 builds).

Receive Segment Coalescing (RSC) in the Hyper-V vSwitch

First of all, you should note the Receive Segment Coalescing (RSC) feature that appeared in Hyper-V on Windows Server 2019/2022 (and Windows 10 1809+). The Receive Segment Coalescing is used at the virtual switch level (vSwitch). RSC allows to reduce CPU load and increase network throughput by combining multiple TCP segments into larger ones. Network performance is improved because large segments are processed faster than many smaller ones.

If RSC support is enabled, it may result in extra network delay in some hardware configurations.

By default, RSC is enabled for all external vSwitches on Windows Server 2019.

You can check if RSC is enabled for virtual switches using the command:

Get-VMSwitch | Select-Object *RSC*

You can disable using RSC for IPv4 traffic on the client network adapter using the following command:

Disable-NetAdapterRsc -Name "Ethernet" -IPv4

Check if the copy speed in a Hyper-V VM has increased after disabling RSC. If the network speed has improved, you can disable RSC on the virtual switch the VM is connected to.

To disable software RSC for a specific virtual switch, run the command:

Set-VMSwitch -Name vSwitchName -EnableSoftwareRsc $false

Or you can completely disable RSC on your Windows host:

netsh int tcp set global rsc=disabled

Virtual Machine Queue (VMQ) Mode in Network Adapter Driver

In some cases, if VMQ (Virtual Machine Queue) is enabled in a network adapter driver of a physical Hyper-V host, it may result in poor network performance in Hyper-V virtual machines. VMQ is a hardware feature and if it is not supported by your hardware but enabled in the driver, it can result in packet loss and increased network latency. The problem is typical to Broadcom Gigabit network adapters and occurs in all Hyper-V versions (Windows Server 2012 R2/2016/2019).

You can disable VMQ in the properties of your network adapter driver.

Or you can display a list of network adapters with VMQ support and their status using PowerShell:

Get-NetAdapterVmq

To disable VMQ for a specific NIC, run the command below (the network adapter will be unavailable for a couple of seconds):

Set-NetAdapterVmq -Name “NICName” -Enabled $False

After disabling VMQ, it is better to restart the host and check the network performance.

Optimize TCP Settings for Hyper-V on Windows Server 2019

Save the current TCP settings on your Hyper-V host and apply new settings that will make TCP settings in Windows Server 2019 almost similar to those of Windows Server 2016.

Save the current settings:

Get-NetTCPSetting -SettingName Datacenter,DatacenterCustom,InternetCustom,Internet|select SettingName,CongestionProvider,CwndRestart,ForceWS|Export-csv c:\backup\ws2019_network_stack_settings_nettcp_backup.csv

Apply new NetTCP settings for LAN:

Set-NetTCPSetting -SettingName DatacenterCustom,Datacenter -CongestionProvider DCTCP
Set-NetTCPSetting -SettingName DatacenterCustom,Datacenter -CwndRestart True
Set-NetTCPSetting -SettingName DatacenterCustom,Datacenter -ForceWS Disabled

For WAN:

Set-NetTCPSetting -SettingName InternetCustom,Internet -CongestionProvider CTCP
Set-NetTCPSetting -SettingName InternetCustom,Internet -DelayedAckTimeoutMs 50
Set-NetTCPSetting -SettingName InternetCustom,Internet -ForceWS Disabled

Disable network RSS and RSC network optimization methods at the TCP stack level:

netsh int tcp show global
netsh int tcp set global RSS=Disabled
netsh int tcp set global RSC=Disabled

or on the NIC level:

Get-NetAdapter | Set-NetAdapterAdvancedProperty -DisplayName "Recv Segment Coalescing (IPv4)" -DisplayValue "Disabled" -NoRestart
Get-NetAdapter | Set-NetAdapterAdvancedProperty -DisplayName "Recv Segment Coalescing (IPv6)" -DisplayValue "Disabled" -NoRestart
Get-NetAdapter | Set-NetAdapterAdvancedProperty -DisplayName "Receive Side Scaling" -DisplayValue "Disabled" –NoRestart

Disable vRSS for all VMs:

Get-VM | Set-VMNetworkAdapter -VrssEnabled $FALSE

Disable Large Send Offload (LSO) on NICs:
Get-NetAdapter | Set-NetAdapterAdvancedProperty -DisplayName "Large Send Offload Version 2 (IPv4)" -DisplayValue "Disabled" -NoRestart
Get-NetAdapter | Set-NetAdapterAdvancedProperty -DisplayName "Large Send Offload Version 2 (IPv6)" -DisplayValue "Disabled" -NoRestart
Get-NetAdapter | Restart-NetAdapter

These TCP stack settings will make Windows Server 2019 network protocol settings similar to those of previous Windows Server versions.