data protection raid

Chapter Objective

After completing this chapter, you will be able to:

Describe what is RAID and the needs it addresses

Describe the concepts upon which RAID is built

Define and compare RAID levels

Recommend the use of the common RAID levels based on performance and

availability considerations

Explain factors impacting disk drive performance

Data Protection: RAID

Data Protection: RAIDWhy RAID

• Performance limitation of disk drive• An individual drive has a certain life expectancy

– Measured in MTBF – Example

• If the MTBF of a drive is 750,000 hours, and there are 1000 drives in the array, then the MTBF of the array becomes 750,000 /1000, or 750 hours

• RAID was introduced to mitigate this problem• RAID provides:

– Increase capacity– Higher availability – Increased performance


RAID Array Components

RAIDControllerRAIDController

Hard Disks

Logical Array

Physical Array

RAID Array

Host

Data Protection: RAIDRAID Implementations

• Hardware (usually a specialized disk controller card)– Controls all drives attached to it– Array(s) appear to host operating system as a regular disk drive– Provided with administrative software

• Software – Runs as part of the operating system – Performance is dependent on CPU workload– Does not support all RAID levels

Data Protection: RAIDRAID Levels

• 0 Striped array with no fault tolerance• 1 Disk mirroring • Nested RAID (i.e., 1 + 0, 0 + 1, etc.) • 3 Parallel access array with dedicated parity disk• 4 Striped array with independent disks and a dedicated parity disk• 5 Striped array with independent disks and distributed parity• 6 Striped array with independent disks and dual distributed parity

Data Protection: RAIDData Organization: Striping

Stripe 1

Stripe 2

Strips

Strip 1=64KB

Stripe=192KB

Strip

Stripe

Strip 2=64KB Strip 3=64KB

Data Protection: RAIDRAID 0

1

95

2

106

3

117

0

Host


Data Protection: RAIDRAID 1

Block 1Block 1 Block 1Block 1Block 1Block 1Block 0Block 0Block 0Block 0

Host

Block 0Block 0 RAIDControllerRAIDController


Host

Block 5Block 5Block 4Block 4Block 2Block 2Block 1Block 1

Nested RAID – 0+1 (Mirrored Stripe)

Block 5Block 5

Block 4Block 4

Block 2Block 2

RAID 0

Block 1Block 1

RAID 1

Block 0Block 0Block 3Block 3 RAIDControllerRAIDController

Data Protection: RAIDNested RAID – 0+1 (Striping and Mirroring)


Block 5Block 5

Block 4Block 4

Block 2Block 2

RAID 0

Block 1Block 1

RAID 1

Block 5Block 5

Block 4Block 4

Block 2Block 2

Block 1Block 1

Block 5Block 5

Block 4Block 4

Block 2Block 2

Block 1Block 1

Host


Host

Nested RAID – 1+0 (Mirroring and Striping)

Block 5Block 5

Block 5Block 5

Block 2Block 2

RAID 1Block 0Block 0Block 0Block 0

Block 2Block 2

RAID 0

Block 3Block 3Block 3Block 3 RAIDControllerRAIDController

Block 1Block 1

Block 1Block 1

Block 4Block 4

Block 4Block 4


Host

Nested RAID – 1+0 (Mirroring and Striping)


RAID 1

Block 1Block 1

Block 1Block 1

RAID 0

Block 4Block 4

Block 4Block 4 Block 5Block 5

Block 5Block 5

Block 2Block 2

Block 2Block 2Block 1Block 1

Block 4Block 4

Data Protection: RAIDRAID Redundancy: Parity

Parity Disk

1

95

3

117

0

0 1 2 34 5 6 7

4

6

1

7

18

Host


Parity calculation 4 + 6 + 1 + 7 = 18The middle drive fails:

4 + 6 + ? + 7 = 18? = 18 – 4 – 6 – 7? = 1

?


Host


Block 1Block 1

Block 2Block 2

Block 3Block 3

P 0 1 2 3

Block 0Block 0Block Block

ParityGenerated

RAID 3


Host

Block 0Block 0

P 0 1 2 3P 0 1 2 3

P 4 5 6 7P 4 5 6 7


P 0 1 2 3

Block 0Block 0

Block 1Block 1

Block 5Block 5

Block 2Block 2

Block 6Block 6

Block 3Block 3

ParityGenerated

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 4Block 4

Block 7Block 7

RAID 4


Host

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 7Block 7


P 0 1 2 3

Block 0Block 4Block 0

Block 1Block 1

Block 5Block 5

Block 2Block 2

Block 6Block 6

Block 3Block 3

ParityGenerated

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 4Block 4

P 4 5 6 7P 4 5 6 7P 4 5 6 7P 4 5 6 7

Block 4Block 4

P 4 5 6 7

Block 4ParityGenerated

RAID 5

Data Protection: RAIDRAID 6 – Dual Parity RAID

• Two disk failures in a RAID set leads to data unavailability and data loss in single-parity schemes, such as RAID-3, 4, and 5

• Increasing number of drives in an array and increasing drive capacity leads to a higher probability of two disks failing in a RAID set

• RAID-6 protects against two disk failures by maintaining two parities– Horizontal parity which is the same as RAID-5 parity– Diagonal parity is calculated by taking diagonal sets of data blocks from the

RAID set members• Even-Odd, and Reed-Solomon are two commonly used algorithms for

calculating parity in RAID-6


RAID MinDisks

Storage Efficiency % Cost Read Performance Write Performance

0 2 100 Low

Very good for both random and sequential

readVery good

1 2 50 HighGood

Better than a single disk

GoodSlower than a single disk, as every write

must be committed to two disks

3 3

(n-1)*100/nwhere n= number of

disksModerate

Good for random reads and very good for sequential reads

Poor to fair for small random writesGood for large,

sequential writes

5 3


disksModerate

Very good for random reads

Good for sequential reads

Fair for random writeSlower due to parity

overhead Fair to good for

sequential writes

6 4


disks

Moderate but more

than RAID 5

Very good for random reads

Good for sequential reads

Good for small, random writes

(has write penalty)

1+0and0+1

4 50 High Very good Good

RAID Comparison


• Small (less than element size) write on RAID 5• Ep = E1 + E2 + E3 + E4 (XOR operations)• If parity is valid, then: Ep new = Ep old – E4 old + E4 new (XOR operations)

– 2 disk reads and 2 disk writes

• Parity Vs Mirroring– Reading, calculating and writing parity segment introduces penalty to every write operation– Parity RAID penalty manifests due to slower cache flushes– Increased load in writes can cause contention and can cause slower read response times

Ep new

RAID Controller

2 XOR

+-= E4 oldEp old E4 new

RAID Impacts on Performance

P0 D1 D2 D3 D4


RAID Penalty Exercise

• Total IOPS at peak workload is 1200• Read/Write ratio 2:1• Calculate IOPS requirement at peak activity for

– RAID 1/0– RAID 5



Hot Spares

Data Protection: RAIDSummary

Key points covered in this chapter:• What RAID is and the needs it addresses• The concepts upon which RAID is built• Some commonly implemented RAID levels

data protection raid

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