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Slow Network on Hyper-V Guests on Windows Server 2019

A couple weeks ago I setup a Hyper-V Server based on Server 2019 offsite. I created the needed VMs and everything was looking good so far. Some days later users began to report very, very slow connections to the fileserver which was a VM running on the mentioned host.

Symptoms

  • Network very slow from PCs to VMs
  • Network OK from PCs to Host
  • Network OK from Host to VMs
  • Network OK from VMs to VMs

Why

Microsoft introduced a new feature in Server 2019 called "Receive Segment Coalescing" (RSC).

Apparently it is used for offloading tasks from the CPU to the network card. It strips the headers from packets and combines the payload of those into one packet.

It is enabled by default on all vSwitches on Windows Server 2019. Of course it doesn't work and just causes problems isn't supported on some hardware configurations.

Solution

Check if RSC is enabled:

Get-VMSwitch | Select-Object name,*RSC*

Disable RSC on a specific vSwitch:

Set-VMSwitch -Name vSwitch1 -EnableSoftwareRsc $false



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