1. Redundant Array of Independent Disks

2.  Many systems today need to store many terabytes of data.  Don’t want to use single, large disk  too expensive  failures could be catastrophic  Would prefer to use many smaller disks.

3.  is a storage technology.  was first defined by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987.  is the organization of multiple disks into a large, high performance logical disk.

4.  An array of multiple disks accessed in parallel will give greater throughput than a single disk.  Redundant data on multiple disks provides fault tolerance.

5.  Striping  Redundancy

6.  Take file data and map it to different disks  Allows for reading data in parallel file datablock 1block 0block 2block 3 Disk 0Disk 1Disk 2Disk 3

7.  In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe.  Data redundancy occurs in database systems which have a data that is repeated in two or more disks.

8.  A number of standard schemes have evolved which are referred to as levels.  There were five RAID levels originally conceived  Other kinds have been proposed in literature  Level 2 and 4 are not commercially available

9.  Break a file into blocks of data  Stripe the blocks across disks in the system  provides no redundancy or error detection  important to consider because lots of disks means low Mean Time To Failure (MTTF)

11.  A complete file is stored on a single disk  A second disk contains an exact copy of the file  Provides complete redundancy of data  Most expensive RAID implementation  requires twice as much storage space

12.  RAID 2 implements bit striping with ECC  Error correction code (Hamming code) allows for correction of a single bit error  is not as efficient as other RAID levels and is not generally used.

13.  Data is striped so each sequential byte is on a different drive  Parity is calculated across corresponding bytes and stored on a dedicated parity drive.  It requires only one disk for parity data.  RAID 3 suffers from a write bottleneck.

14.  Similar to RAID 3.  It employs striped data in much larger blocks or segments.  Not used commercially.

15.  Distribution of the parity strip to avoid the bottle neck.  Best of all worlds  read and write performance close to that of RAID Level-1  requires as much disk space as Levels-3,4

16.  Combine two levels and get the advantages from both.  Examples: 0+1, 1+0, 0+3, 3+0, 0+5, 5+0, 1+5, and 5+1.

17.  Today, RAID is found everywhere---  In operating system software.  A stand-alone controller providing advanced data integrity in high-end storage area networks.  Laptops, as well as desktops, workstations, servers, and external enclosures with a larger number of hard disk drives.  RAID is even included in TV set top boxes or personal storage devices.