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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.
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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
Disk 0 Disk 1 Disk 2 Disk 3 Striping • Take file data and map it to different disks • Allows for reading data in parallel file data block 0 block 1 block 2 block 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 databasesystems 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)
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 2implements 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. • Astand-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.
