RAID levels defined

RAID 0

Characteristics: Disk spreads data across two or more disk drives to improve I/O performance by performing parallel I/O. One/nth of the data is on each of the disk drives where "n" is the number of disks.

Application: Provides high performance for reads and writes. However, there is no data redundancy. RAID 0 by itself should only be used for applications that can tolerate loss of access to data and data that can reproduced from other sources.

RAID 1

Characteristics: Disk mirroring provides data protection and enhanced read performance. RAID 1 mirrors data across two or more disks so that each disk is identical to each other. RAID 1 utilizes n+n protection doubling the number of devices needed.

Application: Read intensive OLTP and other transactional data for high performance and high availability. Other applications that can benefit from RAID 1 include email, operating systems, application files, and read and random intensive environments.

RAID 0+1

Characteristics: Stripe and mirroring of data to provide high performance (stripe) and availability (mirroring) using n+n number of devices. The loss of a disk drive does not impact performance or availability, as would be the case with RAID 0 while the disk stripe enhances performance.

Application: OLTP and I/O-intensive applications requiring high performance and availability. This includes transaction logs, journal files and database indices where the cost measurement is based on dollar per I/O, compared to dollar per unit of storage.

RAID 1+0 (RAID 10)

Characteristics: Similar to RAID 0+1, mirrors and stripes data to provide high performance (stripe) and availability (mirroring) using n+n number of devices. The differences is striping groups of disks together and then mirroring the stripe groups.

Application: OLTP and I/O-intensive applications requiring high performance and availability. This includes transaction logs, journal files and database indices where the cost measurement is based on dollar per I/O, compared to dollar per unit of storage.

RAID 3

Characteristics: Stripes with dedicated parity at the byte level and has a single dedicated disk drive that stores parity information using an n+1 approach in terms of the number of devices needed.

Application: This provides good performance for video imaging, geophysics, life sciences, or other sequential processing applications. RAID 3, however, is not well suited for applications requiring concurrent I/O operations from multiple users or I/O streams.

RAID 4

Characteristics: Same as RAID 3, but with block-level parity protection.

Application: Using read and write cache is well suited with file serving environments.

RAID 5

Characteristics: Disk striping and rotating parity protection using n+1 number of components provides good availability with good read performance for multiple concurrent users and I/O streams. Using a hot spare disk drive, data can be reconstructed (drive rebuild) to protect against a second failure once completed.

Application: Reduces the number of components required while providing good availability, good performance for reads, write performance is impacted if write cache not utilized. Good applications for RAID 5 include reference data, read intensive database tables, general file sharing and Web applications.

RAID 6

Characteristics: Disk striping with rotating parity using dual-parity drives intended to reduce data availability exposure during a disk drive rebuild, particularly when using larger capacity Fibre Channel and SATA disk drives. The issue with RAID 6, and any multidrive parity scheme, is performance overheard for parity calculations when writing data or rebuilding from a failed disk drive.

Application: In general, if you are looking for high-performance reads and writes, look at using smaller disk drives and avoiding RAID 6. If on the other hand you are looking to store large amounts of data where rebuilds can occur in the background, RAID 5 and RAID 6 can be a fit when properly configured to your application needs.

Check out the entire RAID handbook.

About the author:
Greg Schulz is founder and senior analyst with the IT infrastructure analyst and consulting firm StorageIO Group. Greg is also the author and illustrator of Resilient Storage Networks (Elsevier) and has contributed material to Storage magazine and other TechTarget sites.