When I was presenting some sessions at Microsoft Tech-Ed 2000 in Orlando the week before last, one question came up several times: Can I use network-attached storage (NAS) with Exchange Server? The topic warrants further discussion.

NAS technology is not all that new. Several vendors have been selling this technology for several years, and others have announced offerings that are in the works. However, the recent hype about storage area networks (SANs) seems to have also generated interest in NAS. SANs and NAS are often confused. In a nutshell, NAS is the implementation of a disk block protocol (e.g., SCSI) over a common network protocol, such as TCP/IP. Some implementations actually use common network file access protocols such as NFS. You might be familiar with Network Appliance’s Filer, an NAS product that lets servers access their storage over a LAN connection. Essentially, the Filer device is a storage cabinet with an embedded processor and a "lite" OS that exposes disk units to network-based clients (which are usually servers themselves). Clients that access NAS-based storage must also run some sort of redirector or client software that lets the OS see the NAS-based disk as locally attached storage. From this point, the application simply accesses NAS-based storage as if it were local to the server. The downside of NAS-based storage is that the data transfer burden is shifted from the storage bus (SCSI, fibre channel) to the network, and the overhead of the underlying transport protocol is added to storage data transfer.

Although effective for applications such as file serving and excellent in terms of centralized management or backup and restore, NAS presents some potential pitfalls for applications such as Exchange Server. Implementing a disk protocol over IP and a dedicated LAN is relatively effective for a file server, but SCSI over IP can’t deliver the high I/Os per second that Exchange Server demands when supporting large user loads. Because disk I/O is the key to Exchange Server performance, Microsoft officially does NOT support NAS with Exchange Server. However, many organizations use NAS for their Exchange deployments and are quite satisfied. These organizations usually tout the great management, heterogeneous host support, and disaster recovery capabilities that NAS can bring to Exchange environments.

For centralized Exchange deployments supporting small user populations (500 users or less), NAS can be a worthwhile alternative to direct or SAN-based storage. However, if you're concerned about performance for servers with large populations, or if the fact that Microsoft doesn't support NAS with Exchange is important to you, stick to a SAN-based alternative for your future storage needs.