Poor performance for VMs that run with an emulated mode storage stack on Windows Server 2012 R2

Applies to: ✔️ Windows VMs

Original KB number:   4056345

Symptoms

Virtual machines (VMs) that are running Windows Server 2012 R2 may experience poor performance if the VM storage stack is in emulated mode. Additionally, you may experience a severe decrease in local resource VHD (temp disk) I/O performance.

Cause

This issue occurs because of a recently discovered bug in Windows that's exposed when the image that's being provisioned has phantom IDE devices. When this issue occurs, the storflt driver is uninstalled from the VM  storage device stack unexpectedly. This causes the VM  storage stack to use the slower path that uses emulated mode each time the VM restarts.

The driver remains uninstalled and isn't reinstalled during successive restarts.

Resolution

This issue is resolved by update 4057903 that is available through Windows Update. For more information, see Hyper-V integration components update for Windows virtual machines.

Contact us for help

If you have questions or need help, create a support request, or ask Azure community support. You can also submit product feedback to Azure feedback community.