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Best Practices for Drive-Letter Assignments on a Server Cluster

Article ID: 318534 - View products that this article applies to.

This article was previously published under Q318534

SUMMARY

This article describes best practices for assigning drive letters to disks on the shared bus on a server cluster.

When a Host Bus Adapter (HBA) driver is loaded and its bus is enumerated, Windows detects and sequentially assigns the first available drive letters to all disks that contain a valid file system. When you configure the shared disk on a server cluster, it is a good idea to assign drive letters statically to the disks on the shared bus. The drive letters should not start with the next available letter. Instead, leave several free drive letters between the local disks and the shared disks. For example, start with drive Q as the quorum disk and then use drives R and S for the data disks. Another method is to start with drive Z as the quorum disk and then work backward through the alphabet with drives X and Y as data disks. Additionally, you might want to consider labeling the drives in case the drive letters are lost. Using labels makes it easier to determine what the drive letter was. For example, a drive label of "DriveR" makes it easy to determine that this drive was drive letter R.

It is a good idea to set static drive letters for the disk on the shared bus and leave free letters between the local disk and the shared disk because:

Adding additional disks to the local nodes (a passive node) may cause the drive letters of the shared disk to be revised up by one letter.
Adding disks to the local node may cause a discontinuous flow in the drive lettering and cause confusion.
Mapping a network drive may conflict with the drive letters on the shared disk.

For example, assume a server cluster with the following configuration:

Each cluster node has one local disk (drive C).
There is one shared disk (drive D).
Node A owns the shared disk (drive D).
You are logged on to node B.

While you are logged on to node B, you add a local disk. This disk may be assigned drive letter D because node B does not currently own drive D. Drive D cannot then fail over to node B because a duplicate drive letter exists. If the administrator then changes the new local disk to drive letter E, there is a discontinuous flow of drive letters which may cause confusion (drive C is a local disk, drive D is a shared disk, and drive E is a local disk). If the administrator maps a network drive on node B, the default drive letter is D. If drive D fails over to node B, access to the mapped network drive is no longer available.