Data Protection: RAID

Chapter 3

Presented by:

Anupam Mittal

Data protection: Concept of RAID and its Components

Data Protection: RAID - 2

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


Performance limitation of a single drive disk drive◦ Limited Capacity◦ Limited access speed

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 Data Protection: RAID - 4

RAID Arrays - 5

RAIDController

RAIDController

RAID Array

Host


RAIDController

RAIDController

Hard Disks

Logical Array

Physical Array

RAID Array

Host

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


0 Striped array with no fault tolerance 1 Disk mirroring 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 Nested RAID (i.e., 1 + 0, 0 + 1, etc.)


© 2008 EMC Corporation. All rights reserved. RAID Arrays - 9

RAID Redundancy: Parity

Parity Disk

0

84

1

95

2

106

3

117

0 1 2 3

8 9 10 114 5 6 7

RAIDController

RAIDController

Host


Parity Calculation

Parity

Data

Data

Data

Data

4

2

3

5

14

5 + 3 + 4 + 2 = 14

The middle drive fails:

5 + 3 + ? + 2 = 14

? = 14 – 5 – 3 – 2

? = 4

RAID Array

© 2008 EMC Corporation. All rights reserved.

Lecture 8, 9, 10

Different RAID levels and their suitability for different application environments: RAID 0, RAID 1

RAID Arrays - 11


Stripe 1

Stripe 2

Strips

Strip 1 Strip 2 Strip 3

Stripe

Strips

Stripes

Strip 3Strip 2Strip 1

Stripe 1


1

95

2

106

3

117

0

Host

RAIDController

RAIDController


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

Host

Block 0Block 0 RAIDController

RAIDController


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


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

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

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

Benefits are identical under normal operations

Rebuild operations are very different◦ RAID 1+0 uses a mirrored pair – only 1 disk is

rebuilt if a disk fails◦ RAID 0+1 if a single drive fails, the entire stripe is

faulted RAID is 0+1 is a poorer solution and is less common

RAID Arrays - 19

RAID Arrays - 20Parity Disk

0

84

1

95

2

106

3

117

0 1 2 3

8 9 10 114 5 6 7

RAIDController

RAIDController

Host


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 4 – Striping with Dedicated Parity Disk

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

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


Hardware (usually a specialized disk controller card)◦ Controls all drives attached to it◦ Performs all RAID-related functions, including volume

management◦ Array(s) appear to the host operating system as a regular disk

drive◦ Dedicated cache to improve performance◦ Generally provides some type of administrative software

Software ◦ Generally runs as part of the operating system ◦ Volume management performed by the server◦ Provides more flexibility for hardware, which can reduce the cost◦ Performance is dependent on CPU load◦ Has limited functionality

RAID Arrays - 26

Comparison of RAID Levels


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

P0 D1 D2 D3 D4


RAIDController

RAIDController

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?


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Data Protection: RAID