Short for Redundant Array of Independent (or Inexpensive) Disks, RAID is a virtual collection of two or more disk drives that enables a degree of fault tolerance and performance. RAID disk drives are used frequently on servers or workstations and are not generally used on personal computers.
There are a number of different RAID levels. The most common levels used widely today are:
RAID-0 – Striped Disk Array with no Fault Tolerance:
RAID-0 provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This provides high performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
RAID-1 – Disk Mirroring:
RAID-1 provides disk mirroring, a technique in which data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service. Disk mirroring is used commonly in on-line database systems where it's critical that the data be accessible at all times.
RAID-5 – Striping with Distributed Parity:
RAID-5 provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID because only one drive is needed to store the stripe error correction information.
RAID-6– Striping with Dual Parity:
RAID 6 provides data redundancy by using data striping in combination with parity information. Similar to RAID 5, the parity is distributed within each stripe. RAID 6, however, uses an additional physical disk to maintain parity, such that each stripe in the disk group maintains two disk blocks with parity information. The additional parity provides data protection in the event of two disk failures.
RAID-10 – A Stripe of Mirrors:
RAID-10 is a combination of levels 1 and 0. Multiple RAID-1 mirrors are created and a RAID-0 stripe is created over them.
RAID-50 – Striping of Distributed Parity Arrays:
RAID 50 is a combination of RAID 5 and RAID 0. A RAID 5 set must have at least three disks. RAID 50 strips data across each RAID 5 subset. RAID 50 provides a higher degree of fault tolerance since 1 drive per RAID 5 set may fail without data being lost. A performance increase over RAID 5 may be realized depending on the configuration due to fewer disks reads per parity calculation.
RAID-60 – Striping of Dual Parity Arrays:
RAID 60 is striping over more than one span of physical disks that are configured as a RAID 6. RAID 60 strips data across each RAID 6 subset. RAID 60 provides a higher degree of fault tolerance since 2 drives per RAID 6 set may fail without data being lost. A performance increase over RAID 6 may be realized depending on the configuration due to fewer disks reads per parity calculation.
For a more detailed discussion of RAID Levels, advantages and disadvantages, please see: http://support.dell.com/support/edocs/storage/Storlink/index.htm