Notice: This website is an unofficial Microsoft Knowledge Base (hereinafter KB) archive and is intended to provide a reliable access to deleted content from Microsoft KB. All KB articles are owned by Microsoft Corporation. Read full disclaimer for more details.

Hyper-V pass-through disks are not accessible after system is restarted


View products that this article applies to.

Symptoms

Consider the following scenario:

You have a Hyper-V Host, which is connected to SAN storage, where the disks which are connected via Fiber Channel or iSCSI and are configured as Hyper-V pass through LUNs to make them accessible directly to the Hyper-V guests.

In this scenario, when the host machine is rebooted, some or all of the pass-through disks may be inaccessible, and the guests are unable to boot if the LUN was a boot LUN, or unable to access data where it was a data volume.

↑ Back to the top


Cause

This behavior can occur when all of the following happen:
  • Hyper-V is configured to pass through LUNs to Hyper-V guests.
  • A change on the array changes the PNP instance ID of the device, making it appear as a new device.
  • As a result of the PNP instance ID changing, the disk order changes.
  • As a result of the disk order changes, the LUNs can no longer be passed through.
Conclusively determining the cause of this can be difficult. In order to determine if the disk ordering changes were the result of a PNP instance ID change it would be necessary to have access to registry data from a backup of the machine in question before the issue occurred to compare the data with the current copy.

For example, take the case below

Note:  the GUID and vendor ID have been modified to show a generic example, where on two different dates, the same GUID has surfaced as different device ID’s.

Current version (below)
 

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE43

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE45

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE47

 
Original version from backup (below):
 

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE17

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE15

SCSI\DISK&VEN_DISK_DEVICE&\{11111111-1111-1111-1111111111111111}&DISK_DEVICE13

 

↑ Back to the top


Resolution

To resolve this issue contact your storage hardware vendor, to determine the cause of PNP instance ID changing. The most common occurrence of this inadvertent change is during a firmware update to an array or array controller.

↑ Back to the top


More Information

For Microsoft Windows Server 2008 R2, the PNP Instance ID was introduced as a method for maintaining disk ordering across reboots. This functionality depends on the device continuing to appear as the same device across reboots. If the PNP Instance ID changes as a result of a hardware change, then it makes the LUN surfaced to Windows appear as a new device, and the disk order is not maintained. 

↑ Back to the top


Keywords: kb

↑ Back to the top

Article Info
Article ID : 2423835
Revision : 1
Created on : 1/7/2017
Published on : 6/6/2014
Exists online : False
Views : 138