Split Mirror, SAN, and NAS

The split-mirror backup and restore process requires the kind of hardware and software that currently only Storage Area Network (SAN) vendors provide. A SAN is a specialized set of networked storage devices. Typically, you implement SAN technology through Fibre Channel, a specialized networking technology that provides high-speed data transfer to external data storage. A Fibre Channel connection is stable and robust enough to support external storage of database data.

To connect to SAN storage, you place Fibre Channel network cards in your database server, connect them to a Fibre Channel switch, then use the SAN software utilities to configure storage volumes for your server. The server gets a direct, exclusive connection to those volumes through the Fibre Channel switch, and the SAN volumes appear to SQL Server as local drive volumes.

The real advantage of using a SAN for database storage is that a SAN can hold larger data volumes than you could ever put on a server. Also, SAN software makes managing and configuring large amounts of disk storage easier.

SANs are often contrasted with less expensive Network Attached Storage (NAS) devices. SANs provide direct and exclusive access to data storage through a private Fibre Channel networking system, whereas NAS devices generally attach to a common network for the purpose of sharing files. However, the line between them is blurring: Several storage-industry initiatives are attempting to bridge the gap between SAN and NAS devices.

Because NAS storage is less expensive than SAN, it's natural to consider NAS devices for storage of large databases. Currently, however, Microsoft provides only limited support for SQL Server 2000 data on NAS systems, and no support for clustered SQL Servers. If you're considering placing SQL Server databases on a NAS system, see the Microsoft article "INF: Support for Network Database Files" (http://support.microsoft.com/support/kb/articles/q304/2/61.asp). In it, Microsoft recommends that, in general, you store SQL Server data only on local disk subsystems or on a SAN.