EMC2 (3)

Data Protection: RAID

Chapter 3

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Chapter Objectives 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

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Data Protection: RAID - 2

Why RAID  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 © 2009 EMC Corporation. All rights reserved.

Data Protection: RAID - 3

RAID - Redundant Array of Independent Disks

RAID Controller

Host RAID Array

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RAID Arrays - 4

RAID Array Components Physical Array

Logical Array

RAID Controller

Hard Disks

Host RAID Array

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Data Protection: RAID - 5

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

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Data Protection: RAID - 6

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.) © 2009 EMC Corporation. All rights reserved.

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RAID Redundancy: Parity 0 4 8 1 5 9 RAID Controller

2 6 10 3 7 11

Host

0123 4567 8 9 10 11

Parity Disk © 2008 EMC Corporation. All rights reserved.

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Parity Calculation 5 + 3 + 4 + 2 = 14

The middle drive fails:

5

Data

3

Data

4

Data

2

Data

5 + 3 + ? + 2 = 14 ? = 14 – 5 – 3 – 2 ?=4

14

Parity

RAID Array © 2008 EMC Corporation. All rights reserved.

RAID Arrays - 9

Data Organization: Striping Stripes

Stripe 1 Strip 2

Strip 1

Strip 3

Strips

Stripe

Strip 1

Strip 2

Strip 3

Stripe 1 Stripe 2 Strips © 2009 EMC Corporation. All rights reserved.

Data Protection: RAID - 10

RAID 0 – Striped Array with no Fault Tolerance 0

1 5 9 RAID Controller

Host

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2 6 10 3 7 11

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RAID 1 – Disk Mirroring

Block 0 1

RAID Block 0 1 Controller

Host

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Nested RAID – 0+1 (Striping and Mirroring) RAID 1

Block 0 Block 2

Block 0 3 2 1

RAID Controller

RAID 0

Block 1 Host

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Block 3

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Nested RAID – 0+1 (Striping and Mirroring) RAID 1

Block 0

Block 0

Block 2

Block 2

RAID Controller

Host

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RAID 0

Block 1

Block 1

Block 3

Block 3

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Nested RAID – 1+0 (Mirroring and Striping) RAID 0

Block 1 Block 3

Block 2 0

RAID Controller

RAID 1

Block 1 Host

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Block 3

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Nested RAID – 1+0 (Mirroring and Striping) RAID 0

Block 0

Block 1

Block 2

Block 3

RAID Controller

Host

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RAID 1

Block 0

Block 1

Block 2

Block 3

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RAID 0+1 vs. RAID 1+0  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

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RAID Arrays - 17

RAID Redundancy: Parity 0 4 8 1 5 9 RAID Controller

2 6 10 3 7 11

Host

0123 4567 8 9 10 11

Parity Disk © 2009 EMC Corporation. All rights reserved.

RAID Arrays - 18

RAID Redundancy: Parity 0

4 1 6 5 9 RAID Controller

Host

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

4 + 6 + ? + 7 = 18

1 ?

3 7 7 11 0123 4 518 67

? = 18 – 4 – 6 – 7 ?=1 © 2009 EMC Corporation. All rights reserved.

Parity Disk Data Protection: RAID - 19

RAID 3 – Parallel Transfer with Dedicated Parity Disk

Block 0 3 2 1

Host

RAID0 Block Controller Block Parity1 Generated Block 2 Block 3 P0123

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RAID 4 – Striping with Dedicated Parity Disk Block 0 Block 4 Block 1 Block 5

Block 0

Parity RAID0 Block Generated Controller

Block 2 Block 6

P0123 Block 3

Host

Block 7 P0123 P4567

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RAID Arrays - 21

RAID 5 – Independent Disks with Distributed Parity Block 0 Block 4 Block 1 Block 5

Block 0 4

Parity RAID4 Block 0 Generated Controller

Block 2 Block 6

P4 05 16 27 3 Block 3 Host

P4567 P0123 Block 7

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Data Protection: RAID - 22

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 © 2009 EMC Corporation. All rights reserved.

Data Protection: RAID - 23

RAID Implementations  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 © 2009 EMC Corporation. All rights reserved.

RAID Arrays - 24

RAID Comparison RAID

0

1

3

5

Min Disks

2

2

3

3

Storage Efficiency %

100

50 (n-1)*100/n where n= number of disks (n-1)*100/n where n= number of disks

6

4

(n-2)*100/n where n= number of disks

1+0 and 0+1

4

50

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Cost

Low

High

Moderate

Moderate

Read Performance

Write Performance

Very good for both random and sequential read

Very good

Good Better than a single disk

Good Slower than a single disk, as every write must be committed to two disks

Good for random reads and very good for sequential reads

Poor to fair for small random writes Good for large, sequential writes

Very good for random reads Good for sequential reads

Fair for random write Slower due to parity overhead Fair to good for sequential writes

Moderate but more than RAID 5

Very good for random reads Good for sequential reads

Good for small, random writes (has write penalty)

High

Very good

Good

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RAID Impacts on Performance RAID Controller

Ep new

Ep old

=

E4 old

-

+

E4 new

2 XOR

Ep new

Ep old

P0

D1

E4 old

D2

D3

E4 new

D4

 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 © 2009 EMC Corporation. All rights reserved.

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Hot Spares

RAID Controller

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 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?

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