RAID – Redundant Array of Independent Disks

RAID – Redundant Array of Independent Disks

•          RAID concept was developed by University of California at Berkeley in 1987.
•       RAID arrays are used to improve the reliability & performance of Hard disks.
•       RAB (RAID Advisory Board) was formed in 1992 to standardize RAID concepts.

    Standard RAID levels advised by RAB are

     Hybrid (Nested) RAID levels

1.              RAID 10 (1+0)
2.                   RAID 01 (0+1) 

       The most common RAID types used today are explained below

Ø RAID 0 – Disk Striping

§  Spreading the data between multiple drives (at least 2 drives) is called data striping.
§  Increases hard drive access speed (High read/write performance)
§  No redundancy
§  File data is divided into blocks and spread in a fixed order among all the disks in the array.
§  If any one of the physical disks fails, the whole logical volume will fail, resulting in the computer crash and requiring to recover the data from other backups.
§  RAID 0 alone is not used in live data storage.



    RAID 1 – Disk Mirroring / Duplexing        

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•  Data written on one drive is duplicated on another drive, providing excellent fault tolerance is called disk mirroring.
• 2 drives each on a separate RAID controller provides high protection to data. This super drive mirroring technique is called duplexing.
• Half of the storage space only can be used, as the data in 1 hard disk is mirrored to the 2nd one.
•  Requires at least 2 hard disks, but any even number of disks can be used.              

          RAID5 -DISKSTRIPING WITH DISTRIBUTED PARITY

§ Protects data by adding extra information called parity data that can be used to rebuild data if one of the drives fails.
§Distributes data and parity information evenly in all the drives.

§ Requires at least 3 drives but any number of drives higher than 3 can be used.

§ RAID 5 is the most common array used.

§ Uses one drive’s worth of space for parity.

§ Redundant & faster.

RAID 6  -- DISK STRIPING WITH EXTRA PARITY

§  Similar to RAID 5 except parity information is written in 2 different parity schemes to provide even better fault tolerance in case of multiple hard drive failures.

Requires a minimum of 4 hard drives (2 or more for data & 2 for parity)

Gaining popularity in large RAID arrays.

Expensive compared to other arrays.

RAID 10 (RAID 1+0)

·Also called Nested, striped mirrors.

 Minimum 4 hard disks required.

 First, groups of mirrors (RAID 1) are formed & the data is stripped across this mirror groups.

 Fault tolerance is higher than RAID 01 because the RAID will function even one disk fails in any one of the mirrored groups.

Always choose RAID 10 when RAID 01 & RAID 10 are given as choices.

 RAID 01  (RAID 0+1)

•       Also known as Nested, mirrored stripes.
•      Data is stripped in a group of disks & the formed group is mirrored to the other group with the exact number of disks as group 1.
•       Fault tolerance is less compared to RAID 10.