RAID Technology

CS350 Computer Organization

Section 2Larkin Young

Rob Deaderick

Amos Painter

Josh Ellis

Overview

• Background– Term coined by researchers at UC-Berkeley– Redundant Array of Inexpensive Disks

• Basics of RAID– Disk arrays– Logical/physical– Controller (software/hardware)

Techniques/Methods

• Mirroring

• Parity

10101010 XOR 11111111 = 01010101

11111111 XOR 01010101 = 10101010

10101010 XOR 01010101 = 11111111

Techniques/Methods (cont’d)

• Striping

RAID 0

• Uses striping– I/O performance gain– No Data redundancy

• Not fault tolerant

• Not considered “true” RAID

RAID 1

• Uses mirroring– Also known as duplexing

• Fault tolerant• High Disk overhead

– Mirroring typically handled system software

• Simplest RAID design

RAID 2

• Error Detection and Correction

• Parity and Hamming Code– Used to identify and correct errors

• Hamming Code– Uses parity bits to verify data integrity

• Parity bits signify oddness of data bits

– ECC (Error Correction Code)

RAID 2

Characterized by:– “On the fly” data correction

– High Ratio of Data disks to ECC disks

– Not a commercially viable solution

– High Costs Involved

RAID 3: XOR

• Exit Hamming Code, Enter XOR (eXclusive OR)

• XOR in Action:01012 XOR 00112 = 01102

If ????2 XOR 11002 = 01102,

Then 11002 XOR 01102 = ????2 = 10102

Thus we can use XOR results to recover lost data

XOR Logic Table

A XOR B Result

0 0 0

0 1 1

1 0 1

1 1 0

RAID 3: Pros and Cons

Advantages:• High Read/Write Transfer

Rates• Disk failures don’t slow

the system• Low Ratio of Data Disks

to Parity Disks

Disadvantages:• Transaction rate slowed

by Parity Disk• Complex Controller

Design• Software Implementation

– Resource Intensive

RAID 3: Uses

• Video Production

• High-end Video and Image Editing

• Other uses that require high throughput of data

RAID 4• Offshoot of RAID 3• No Striping, Entire files written to individual disks• Parity Data created on a sector-by-sector basis

RAID 4: Pros, Cons, and Uses

Advantages:• Very high read rates

– Multiple files read at once

Uses:• Web Servers, and other

high read, low write situations

Disadvantages:• Very slow write rates

– Even small writes fill up parity write queue

• Inefficient data recovery• Even more Complex

Controller Design than RAID 3

*Has most of the other Advantages and Disadvantages of RAID 3

RAID 5

• Highest Read data transaction rate

• Medium Write data transaction rate

• Most complex controller design

• Used For Server Applications.

RAID 6

• Fault tolerance

• Very complex controller design

• Very poor write performance

RAID 7

• Extremely high cost per MB

• Fast access times

• Improved write performance with increasing number of drives

RAID 10

• Uses multiple (mirrored) RAID 1 in a single array

• Data striped across all mirrored sets

• Very high fault tolerance

• High performance rate

RAID 10

Characterized by:

- each drive duplicated

- high implementation cost

RAID 53

• Consists of a striped array made up of RAID 3 segments

• Very expensive• Achieves high rate of I/O

RAID 0+1

• Acts as a mirrored array whose parts consist of RAID 0 arrays

• Very expensive

• Works well in handling images– very high data transfer rate

RAID 0+1

- Not great reliability

- one disk failure leaves you left with only the characteristics of a striped array (no fault tolerance)

Conclusion

• Grown more complex

• Large price range

• Huge step forward for industry

• Not for the masses yet