RAID (redundant array of independent disks) is a disk system that comprises multiple disk drives (an array) to provide higher performance, reliability, storage capacity, and lower cost. Fault-tolerant arrays are categorized in six RAID levels, 0 through 5. Each level uses a different algorithm to implement fault tolerance.

Although RAID is not a part of Microsoft® SQL Server™ 2000, its implementation can directly affect the way SQL Server performs. RAID levels 0, 1, and 5 are typically used with SQL Server.

Note  RAID is available only on Microsoft Windows NT 4.0 and Microsoft Windows 2000.

A hardware disk array improves I/O performance because I/O functions, such as striping and mirroring, are handled efficiently in firmware. Conversely, an operating system–based RAID offers lower cost but consumes processor cycles. When cost is a consideration and redundancy and high performance are required, Microsoft Windows® NT® stripe sets with parity or Windows 2000 RAID-5 volumes are a good solution.

Data striping (RAID 0) is the RAID configuration with the highest performance, but if one disk fails, all the data on the stripe set becomes inaccessible. A common installation technique for relational database management systems is to configure the database on a RAID 0 drive and then place the transaction log on a mirrored drive (RAID 1). You can get the best disk I/O performance for the database and maintain data recoverability (assuming you perform regular database backups) through a mirrored transaction log.

If data must be quickly recoverable, consider mirroring the transaction log and placing the database on a RAID 5 disk. RAID 5 provides redundancy of all data on the array, allowing a single disk to fail and be replaced in most cases without system downtime. RAID 5 offers lower performance than RAID 0 or RAID 1 but higher reliability and faster recovery.