Slow copy performance to a Hyper-V guest on a host with a Broadcom adapter

If any of the root paths involved in the synchronization are running as guests on a Hyper-V server, it is important to check if the host machine is running with Broadcom network interface cards (NICs). If the host has Broadcom network adapters with Virtual Machine Queue (VMQ) enabled, you will experience significantly reduced network performance depending on driver version.

This issue is outlined in the Microsoft KB article found here. Microsoft states that Broadcom has acknowledged the problem and will be releasing a driver update. The version of the driver included with Windows 2012 R2 server for the Broadcom NetXtreme Gigabit Ethernet 5720 adapter is v15.6.1.3 which still displays the problem. We have tested with version and the problem appears to be largely correct. However, our tests still show better performance with VMQ disabled. There are many machines out there running the out of date driver.

Options to address the Broadcom issue are:

  • Update the driver to the current version for your adapter model
  • Disable VMQ. To do this, launch Device Manager and expand Network Adapters. For the adapters used by the Hyper-V guest, right click and select Properties. On the Advanced tab, scroll down and click on Virtual Machine Queue. Change this setting to Disabled and apply the change. This can interrupt  network access briefly so do this during a maintenance window.