By Nagendra Srivastava

RAID Technology

CONTENT

IntroductionTechniques/methods

Mirroring Striping Parity

Advantages and Disadvantages Uses Conclusion

History

> It first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 to describe a redundant array of inexpensive disks.

>What is RAID - It is a technology that allowed computer users to achieve high levels of storage reliability from low-cost and RAID allows you to store the same data redundantly (in multiple paces) in a balanced any to improve overall performance.

What is RAID

Benefits of RAID>Data loss can be very dangerous for an organization

>RAID technology prevents data loss due to disk failure

>RAID technology can be implemented in hardware or software

>Servers make use of RAID technology

Hardware vs. Software RAID >Software RAID

• Software RAID: run on the server’s CPU• Directly dependent on server CPU performance and load• Occupies host system memory and CPU operation, degrading server performance

>Hardware RAID• Hardware RAID: run on the RAID controller’s CPU• Does not occupy any host system memory. Is not operating system dependent• Host CPU can execute applications while the array adapter's processor simultaneously executes array functions: true hardware multi-tasking

•Redundant Arrays of Independent Drives

•Benefits– Improved data availability– Improved I/O performance– Increased scalability

•Levels supported by Array controllers– RAID 0 – Data striping– RAID 1 – Drive mirroring– RAID 4 – Data guarding– RAID 5 – Distributed data guarding– RAID 6/ADG - Advanced data guarding– RAID 0+ 1 – Mirroring and striping

RAID

                                                

Single Drive and Drive Arrays

Single Drive Drive Arrays

Fixed Disk Drive Array

SingleDrive 1 2 3 4

234

1

1 2 3 4

Drive Array Features

•Data striping across multiple drives

•Multiple channels

•Request processing

Drive Array Organization

Techniques/Methods

Mirroring

Parity

10101010 XOR 11111111 = 01010101

11111111 XOR 01010101 = 10101010

10101010 XOR 01010101 = 11111111

Techniques/Methods (cont’d)

Striping

File divided into chunks (or segments) and then written (striped) across multiple drives

RAID Level 0 — Data Striping

RAID 0

Uses striping I/O performance gain No Data redundancy

Not fault tolerantNot considered “true” RAID

Striping Factor

64KB Host Data

Fixed Striping Factor Based on Drive Sizes

16KB

16KB

16KB

16KB

Data Written to Two or More Separate Mirrored Drives

RAID Level 1 — Drive Mirroring

RAID 1

Uses mirroringAlso known as duplexing

Fault tolerantHigh Disk overhead

Mirroring typically handled system software

Simplest RAID design

Disk 0 is mirrored to disk 2 and disk 1 is mirrored to disk 3.Then disk 0 is striped with disk 1 and disk 2 is striped to disk 3.

RAID Level 0+1

RAID 0+1

RAID 0+1

RAID 1+0 requires an array with four or more physical disks. The disks are mirrored in pairs and data blocks are striped across the mirrored pairs

Advantages Highest read and write performance No loss of data as long as no failed disks are mirrored to any

other failed disk

Disadvantages Expensive and Low disk capacity

Data striped across multiple drives and then its parity sum is calculated and written to a dedicated parity drive

RAID Level 4 — Data Guarding

RAID 4

Disadvantages:

Very slow write rates Even small writes fill up

parity write queue

Inefficient data recovery

Even more Complex Controller Design than RAID 3

Advantages:

Very high read ratesMultiple files read at

once

Uses:

Web Servers, and other high read, low write situations

Data is striped across multiple drives and then its parity sum calculated and striped across multiple drives. Example of 64KB striped across five drives using 4KB chunks.

RAID Level 5 — Distributed Data Guarding

RAID 5

RAID 5 uses a parity data formula to create fault tolerance. In RAID 5 each block of data stripe contains parity data

that is calculated for the other data blocks in that strip. The blocks of parity data are distributed over the physical

disks that make up the logical drive with each physical disk containing only one block of parity data

It is referred to as data guarding.Advantages

High read performance No loss of data if one physical disk fails More usable disk capacity

Disadvantages Relatively low write performance Data loss occurs if a second disk fails before data from the first

failed disk is rebuilt

RAID ADG (Advanced Data Guarding)

>RAID ADG is similar to RAID 5 except this RAID level writes 2 sets of parity stripped across all drives.

>Protects against failure of ANY 2 drives in the array

RAID ADG

A

D

G

P

B

E

PQ

C

PQJ

PQH

K

QF

I

L

RAID ADG (Advanced Data Guarding)

>P = f1(A, B, C) = RAID 5 Parity

>Q = f2(A, B, C) = new ADG Parity

RAID ADG

A

D

G

P

B

E

PQ

C

PQJ

PQH

K

QF

I

L

RAID ADG (Advanced Data Guarding)>If 2 parity drives are selected, the system can sustain failure of ANY 2 drives.

RAID ADG

A

D

G

P

B

E

PQ

C

PQJ

PQH

K

QF

I

LX X

RAID ADG (Advanced Data Guarding) or RAID 6

RAID advanced data guarding sometimes referred to as RAID 6, is similar to RAID 5 in that parity data is generated and stored to protect against data loss caused by physical disk failure

Advantages High read performance High data availability More usable disk capacity

RAID 10

Combining RAID 0 and RAID 1 is often referred to as RAID 10 which offers higher performance than RAID 1 but at much higher cost

Uses multiple (mirrored) RAID 1 in a single arrayData striped across all mirrored setsVery high fault toleranceHigh performance rate

RAID 10

Characterized by:

Each drive duplicated

High implementation cost

Comparing RAID Levels

RAID 0 RAID 1 RAID 5 RAID 10

Read High 2X High High

Write High 1X Medium High

Fault tolerance

No Yes Yes Yes

Disk utilization

High Low High Low

Key problems

Data lost when any disk fails

Use double the disk space

Lower throughput with disk failure

Very expensive, not scalable

Key advantages

High I/O performance

Very high I/O performance

A good overall balance

High reliability with good performance

On-Line Spare

•Replacement for failed drive

•Requires hardware fault tolerance

•Background rebuild process

•Four On-Line Spares maximum (Smart Array controller)

MirroredPair

BeforeFailure

On-LineSpare

MirroredPair

AfterReplacement

On-LineSpare

MirroredPair

DuringFailure

Conclusion

So what have we learned here?

Well we have learned that RAID is not just a bug spray. RAID is a good solution for companies or individuals carving more transfer performance, redundancy and storage capacity in their data storage

systems.

Thanks