RAID 0+1 requires a minimum of four drives

RAID Levels

The Xserve RAID system supports several RAID levels. Each level has a different architecture and provides varying degrees of performance and fault tolerance. Each level has characteristics to achieve maximum performance or redundancy depending on the data environment. Understanding the differences among RAID levels will help you set up your Xserve RAID system to best meet your data performance and security needs.

RAID 0: Data striping without fault tolerance

Also referred to as striping, RAID level 0 is a performance-oriented mapping technique for disk sets. Uniform subsets of the array’s logical volume, called stripes, are mapped in regular sequence to a set’s drives, or elements. Using either independent or parallel access, RAID 0 provides high I/O performance at low cost. But RAID 0 offers no redundancy, so it is not recommended for use with the Xserve RAID system. (RAID 0 can be combined with other techniques to provide data redundancy, regeneration, and reconstruction, however.)

This level of RAID should never be used in mission-critical environments. It may be acceptable as a working environment or “scratch disk.”

RAID 0 is easy to conﬁgure and has a simple structure; it requires a minimum of two drives.

RAID 1: Mirroring

RAID level 1, mirroring, has been used longer than any other RAID level and remains popular because of its simplicity and high levels of reliability and availability. Mirroring uses two drives or multiple drive pairs. Each drive in a pair stores identical data. RAID 1 may use parallel access for high transfer rates but more commonly uses independent access for high transaction rates. RAID 1 provides very high data reliability and improved performance for read-intensive applications, but this level has a high capacity cost because it retains a full copy of your data on each drive in a pair. RAID 1 has the highest Error Checking/Correction (ECC) disk overhead of all RAID variants—100 percent.

Common applications of RAID 1 are accounting and payroll, or any application requiring high data availability. RAID 1 requires a minimum of two drives. In a RAID 1 conﬁguration, the capacity of the smallest drive is the maximum storage area. Additional drives provide more redundancy, and thus more protection, but no additional capacity.

RAID 0+1: High data-transfer performance

RAID 0+1 is a combination of level 0 (striping) and level 1 (mirroring). The RAID controller sets up mirrored pairs of drives and adds striping across the pairs. This RAID level gives the high performance of striping and the reliability of mirrored data, but it has the disadvantage of requiring double the drive capacity and is therefore fairly expensive.