Duplexed Mirror Generation Fails with Insufficient Space

• Microsoft Windows NT Server version 4.0

SYMPTOMS

When you use duplexed SCSI controllers and a software mirror of either the operating system partition or a data partition, you may receive the message "Insufficient space on drive X" where X is the target drive or partition. This can happen when using identical controllers and drives.

CAUSE

When two SCSI controllers are used, the drive geometry is calculated differently for controllers that have BIOS enabled and the identical controller with BIOS disabled.

With a controller that has BIOS enabled, the operating system can query the controller BIOS for drive geometry. The BIOS returns a generic drive geometry translation. This translation will have a cylinder count of less than 1,024 to work with MS-DOS. The number of heads is normally increased to arrive at a correct overall size.

Because the second controller does not have BIOS enabled, all it can provide the operating system is the size of a block (sector size) and the total number of blocks on the device. Windows NT then uses a default translation of 32 tracks per cylinder, 64 heads and adjusts the number of cylinders to arrive at a correct approximate size. Because of the different translations, cylinder boundaries will be in different places on the drives. This often results in a drive's free space being 1 or 2 MB different on the two identical drives.

Internally, Windows NT uses a LARGE_INTEGER (64-bit) number for the number of cylinders and is not constrained by MS-DOS or partition table limitations.

RESOLUTION

To work around this problem, perform one of the following:

• If the SCSI controller supports it, set the translation mode on the primary controller to 32 sectors per track and 64 heads.
• Disable the BIOS on the primary controller, and enable it on the secondary controller. Because creating a mirror does not mirror the master boot record, it may also be necessary to set the active partition byte on the new boot drive with MS-DOS Fdisk.exe or with a sector editor. An alternative is to boot the computer from a Windows NT boot floppy disk. Information on how to create this disk can be found in the Windows NT Resource Kit and the Concepts and Planning Guide.

STATUS

Microsoft has confirmed this to be a problem in Windows NT SUR version 4.0. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.