© 2009 emc corporation. all rights reserved. data protection: raid module 1.3

© 2009 EMC Corporation. All rights reserved.

Data Protection: RAIDData Protection: RAID

Module 1.3

© 2009 EMC Corporation. All rights reserved. Data Protection: RAID - 2

Module objectives

After completing this module, 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


Why 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 100 drives in the

array, then the MTBF of the array becomes 750,000 / 100, or 7,500 hours

RAID was introduced to mitigate this problem

RAID provides: – Increase capacity

– Higher availability

– Increased performance

Redundant Array of Independent DisksRedundant Array of Inexpensive Disks


RAID Array Components

RAIDController

RAIDController

Hard Disks

Logical Array

Physical Array

RAID Array

Host


RAID 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


RAID Controller Card


RAID 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 Organization: Striping

Stripe 1

Stripe 2

Strips

Strip 1 Strip 2 Strip 3

Stripe

Strip

Stripe


RAID 0 (Striped array with no fault tolerance)

1

95

2

106

3

117

0

Host

RAIDController

RAIDController


RAID 1 (Disk mirroring)

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

Host

Block 0Block 0 RAIDController

RAIDController


Nested RAID – 0+1 (Striping and Mirroring)

Block 3Block 3

Block 2Block 2

Block 1Block 1

Host

RAID 0

Block 0Block 0

Block 3Block 3Block 2Block 2Block 1Block 1Block 0Block 0

RAID 1

RAIDController

RAIDController


Nested RAID – 0+1 (Striping and Mirroring)

RAIDController

RAIDController

Block 3Block 3

Block 2Block 2

Block 1Block 1

RAID 0

Block 0Block 0

RAID 1

Block 3Block 3

Block 2Block 2

Block 1Block 1

Block 0Block 0

Block 3Block 3

Block 2Block 2

Block 1Block 1

Block 0Block 0

Host


Host

Nested RAID – 1+0 (Mirroring and Striping)

Block 3Block 3

Block 3Block 3

Block 1Block 1

RAID 1Block 0Block 0Block 0Block 0

Block 1Block 1

RAID 0

Block 2Block 2Block 2Block 2 RAIDController

RAIDController


Host

Nested RAID – 1+0 (Mirroring and Striping)

RAIDController

RAIDController

RAID 1

Block 0Block 0

Block 0Block 0

RAID 0

Block 2Block 2

Block 2Block 2 Block 3Block 3

Block 3Block 3

Block 1Block 1

Block 1Block 1Block 0Block 0

Block 2Block 2


RAID Redundancy: Parity

Parity Disk

1

95

3

117

0

0 1 2 34 5 6 7

4

6

1

7

18

Host

RAIDController

RAIDController

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

4 + 6 + ? + 7 = 18

? = 18 – 4 – 6 – 7

? = 1

?


Host

RAIDController

RAIDController

Block 1Block 1

Block 2Block 2

Block 3Block 3

P 0 1 2 3

Block 0Block 0Block 3Block 3Block 2Block 2Block 1Block 1Block 0Block 0

ParityGenerated

RAID 3 (Parallel access array with dedicated parity disk) (Bit-interleaved Parity)


RAID 4 - Striped array with independent disks and a dedicated parity disk(Block-interleaved Parity)

RAIDController

RAIDController

P 0 1 2 3

Block 0Block 0

Block 0Block 0

Block 4Block 4

Block 1Block 1

Block 5Block 5

Block 2Block 2

Block 6Block 6

Block 3Block 3

Block 7Block 7

P 0 1 2 3P 0 1 2 3

P 4 5 6 7P 4 5 6 7

ParityGenerated

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Host


Host

Block 0Block 0

P 0 1 2 3P 0 1 2 3

Block 7Block 7

RAIDController

RAIDController

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 (Striped array with independent disks and distributed parity)


RAID 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 6 (P+Q)


RAID 6-DP (Diagonal Parity)


RAIDMin

DisksStorage

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 3 & 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

Ep new Ep old E4 old E4 new

+-= 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


Solution

Total IOPS = 1200

Read / Writ ratio 2:1

For RAID 1/0:

(1200x2/3) + (1200x(1/3)x2) = 800 + 800 = 1600 IOPS

For RAID 5:

(1200x2/3) + (1200x(1/3)x4) = 800 + 1600 = 2400 IOPS


RAIDController

RAIDController

Hot Spares


Module Summary

Key points covered in this module:

What RAID is and the needs it addresses

The concepts upon which RAID is built

Some commonly implemented RAID levels


Check Your Knowledge

What is a RAID array?

What benefits do RAID arrays provide?

What methods can be used to provide higher data availability in a RAID array?

What is the primary difference between RAID 3 and RAID 5?

What is advantage of using RAID 6?

What is a hot spare?

© 2009 emc corporation. all rights reserved. data protection: raid module 1.3

Documents

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host block

striping raid controller

raid module

striping block

hours raid

raid redundancy

problem raid