Redundant Array of Independent Disks RAID Mass Storage

Redundant Array of Independent Disks RAID

Mass Storage �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.

What is RAID? �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.

The Need for RAID �An array of multiple disks accessed in parallel will give greater throughput than a single disk. �Redundant data on multiple disks provides fault tolerance.

Two Important Concepts �Striping �Redundancy

Striping �Take file data and map it to different disks �Allows for reading data in parallel file data block 0 Disk 0 block 1 Disk 1 block 2 Disk 2 block 3 Disk 3

Redundancy �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.

Standard Levels �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

RAID 0 �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)

Data Mapping for RAID 0

RAID 1 � 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

RAID 2 �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.

RAID 3 �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.

RAID 4 �Similar to RAID 3. �It employs striped data in much larger blocks or segments. �Not used commercially.

RAID 5 �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

Combinations of different levels �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.

Uses �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.

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