Creating Mirror Fails with Insufficient Disk Space

• Microsoft Windows NT Server versions 3.5, 3.51, and 4.0

SYMPTOMS

When you use duplexed SCSI controllers and a software mirror of either the operating system partition or a data partition, an "Insufficient space on drive X" appears where X is the target drive or partition.

CAUSE

This problem may occur even when you use identical SCSI controllers and drives but whose BIOS is disabled on the second SCSI controller. When two identical SCSI controllers are used, the drive geometry is calculated differently for the controller with the BIOS enabled and the controller with the BIOS disabled.

For 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 1024 to work with MS-DOS. The number of heads is normally increased to arrive at a correct overall size.

When the second SCSI controller does not have its BIOS enabled, it provides the operating system with the size of a block (sector size) and the total number of blocks on the device. Windows NT uses a default translation of 32 tracks per cylinder, 64 heads and adjusts the number of cylinders for a correct approximate size. Due to the different translations, cylinder boundaries are located in different places on the drives. This often results in free space being one or two megabytes (MB) different on the two identical drives or whatever 1 cylinders worth of data is. 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, do one of the following:

• Set the translation mode on the primary controller to 32 sectors per track and 64 heads (if the SCSI controller supports this translation mode).

  -or-

• Disable the BIOS on the primary SCSI controller and enable it on the secondary controller. When you create a mirror, the master boot record is not part of the mirror. Therefore, set the active partition byte on the new boot drive with MS-DOS FDISK.EXE or a disk sector editor. Another method is to boot the computer from a Windows NT boot floppy disk. For more information on how to create this boot floppy disk, refer to the Windows NT Resource Guide or the Windows NT Concepts and Planning Guide.

STATUS

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