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Applies to: ✔️ Windows VMs
Original KB number: 4056345
Summary
This article describes an issue that causes poor performance to occur for VMs that run with an emulated mode storage stack on Windows Server 2012 R2. It also provides a resolution for the issue.
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.