The Magic of Mount Points

Practical Applications and Implications
To more easily understand the utility of mount points, let's look at some specific examples of how they're used and the benefits they provide. These practical examples illustrate the use of volume mount points in Windows Server and Exchange Server environments.

Let's start with a simple example. Suppose that you want to add a second hard disk to your Windows machine. You already have one hard disk (Drive 1) mapped as C, and you don't want to map the second disk (Drive 2) as D. You can get around this problem by adding a mount point to the directory structure of Drive 1 that references Drive 2, as Figure 1 shows. In this example, Drive 2 is configured as a mount point of C:\My Documents\Data. When traversing the directory tree to C:\My Document\Data, the user or application is redirected to the root of Drive 2. This simple technique avoids assigning a drive letter and greatly simplifies directory navigation.

Now let's look at a more complicated, real-world example. Microsoft's Operations and Technology Group (OTG) operates one of the world's largest Exchange deployments, which supports more than 88,000 mailboxes, more than 50TB of storage, and more than 6 million messages per day. As part of an aggressive consolidation project during the past year, OTG decided to use Microsoft Cluster Server (MSCS) clusters for its large Exchange servers. The largest of these configurations is a five-node cluster (four active nodes and one standby node) that supports 16,000 mailboxes (4000 mailboxes per active node).

As you can imagine, this large-scale configuration requires huge amounts of storage consisting of multiple LUNs used as database volumes, log-file volumes, and backup volumes. OTG originally designed this configuration without using volume mount points because mount points weren't supported on clustered shared disks before Windows 2003. The original configuration required 36 drive letters, which rendered the cluster design impossible to implement and deploy in Win2K Server.

However, because Windows 2003 supports the use of mount points on clustered shared disks, OTG was able to reduce the original design's 36 drive letters to 16 drive letters in the Windows 2003­based design. This modification let OTG build and scale the Exchange clusters to meet its requirements for the consolidation project. For Microsoft, volume mount points were a key enabler for a leading-edge Exchange cluster design.

Volume mount points are powerful tools for both simple and complex applications. You can use them to overcome drive-letter limitations or to simplify directory traversal. Whether you're building large-scale cluster configurations or just trying to simplify volume management on your file server, you'll come to appreciate mount points the way the UNIX gurus do.