Write Cache Warnings on Hyper-V Guest after KB2853952

Mark Berry October 10, 2013

Starting in September 2013, Windows Server 2012 R2 machines running as Hyper-V guests started issuing these warnings on reboot:

Log Name:      System
Source:        Disk
Event ID:      32
Level:         Warning
The driver detected that the device \Device\Harddisk0\DR0 has its write cache enabled. Data corruption may occur.

Log Name:      Directory Service
Source:        Microsoft-Windows-ActiveDirectory_DomainService
Event ID:      1539
Level:         Warning
Active Directory Domain Services could not disable the software-based disk write cache on the following hard disk.
Hard disk:
Data might be lost during system failures.

It turns out this started after the patch described in KB2853952 was installed with the September 2013 updates. Before the patch, when a Hyper-V guest tried to disable write caching on a virtual IDE disk, it would report success even though it didn’t really succeed. With the patch, it reports failure.

The background on this is covered in this thread and this blog post. In summary:  it’s a Good Thing that the guest knows write caching can’t be disabled, because then Active Directory falls back to non-cached updating. The 1539 message says "Data might be lost during system failures" but really it means "Active Directory data will NOT be lost during system failures because Active Directory will not use write caching."

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