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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