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Windows Server 2008 R2 hangs at black screen during boot

After configuring a Windows Server 2008 R2 server with an iSCSI SAN storage device, the system may hang during boot at a black screen.
This can occur if the system is using network interface cards (NIC) that trigger a PnpRebalance during boot and Quality of Service (QoS) is enabled in the Networking components.
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.

For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: 322756 ( How to back up and restore the registry in Windows
  1. Boot the system into Safe Mode.
  2. Click Start, click Run, type regedit in the Open box, and then click OK.
  3. Locate the following subkey in the registry, and then click this subkey:
  4. Highlight Start and select Modify... from the Edit menu.
  5. In the Value data: field, change the 1 to 2 and click OK.
  6. Close Regedit and reboot the system.

With an iSCSI target configured for the system, even though we are not booting from the iSCSI target, it causes the networking components to be initialized earlier in the boot process.  In this case, due to the PnpRebalance occurring when the second NIC is initialized, this is triggering a removal of the initial NIC device, which is, at the time, attempting to initialize pacer.sys in the networking stack.  Because this is now occurring earlier in the boot process, the paging path flag has not yet been set on the system disk, so the boot volume  is allowed to be removed during the PnpRebalance. We then see the hang due to pacer.sys blocking the removal of the device, which in turn blocks the PnpRebalance from completing.  While all the components are working as designed, pacer.sys is  not boot safe, in that it requires access to the registry during its initialization.  By setting the Start value for Psched (pacer.sys) to 2, we prevent it from being initialized during this process, thus preventing the hang.
Note This is a "FAST PUBLISH" article created directly from within the Microsoft support organization. The information contained herein is provided as-is in response to emerging issues. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice. See Terms of Use for other considerations.

Article ID: 2601355 - Last Review: 08/22/2011 15:52:00 - Revision: 2.0

  • Windows Server 2008 R2 Enterprise
  • Windows Server 2008 R2 Datacenter
  • Windows Server 2008 R2 Standard
  • KB2601355
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