RAID ARRAYS

Redundant Array of Inexpensive Discs

WHAT IS RAID ARRAYS?RAID is an acronym for Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks)The various types of RAID are data storage schemes that divide and/or replicate data among multiple hard drives

WHY USE RAID?

Improved ReliabilityImproved PerformanceFault ToleranceImproved AvailabilityHigher Data Security

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KEY TERMS Mirroring - the copying of data to more than

one disk Striping - the splitting of data across more

than one disk Parity - a redundancy check that ensures that

the data is protected without having to have a full set of duplicate drives.

Duplexing - an extension of mirroring that is based on the same principle as that technique expect it goes one step further in that it also duplicates the hardware that controls the two hard drives (or sets of hard drives).

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RAID - REDUNDANT ARRAY OF INDEPENDENT DISKS

RAIDController

RAIDController

RAID Array

Host

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RAID COMPONENTS

RAIDController

RAIDController

Logical Array

Logical Array

Physical Array

RAID Array

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DATA ORGANIZATION: STRIPS AND STRIPES

Stripe 1Stripe 2Stripe 3

Strips

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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 Combinations of levels (I.e., 1 + 0, 0 + 1,

etc.)

RAID 0

A striped set of at least two disks without parityThe data is broken down into blocks and each block is written to a separate disk driveBest performance is achieved when data is striped across multiple controllers with only one drive per controller

RAID Arrays

RAID 0 – STRIPED ARRAY WITH NO FAULT TOLERANCE

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RAIDController

RAIDControllerBlock 4Block 4 Block 4Block 4Block 3Block 3 Block 3Block 3Block 2Block 2 Block 2Block 2Block 1Block 1 Block 1Block 1Block 0Block 0 Block 0Block 0

Host

ADVANTAGES OF RAID 0

I/O performance is greatly improved by spreading the I/O load across many channels and drivesNo parity calculation overhead is involvedVery simple designEasy to implement

DISADVANTAGES OF RAID 0

Not a "True" RAID because it is NOT fault-tolerant The failure of just one drive will result in all data in an array being lostShould never be used in mission critical environments

RAID Arrays

RAID 1 – DISK MIRRORING

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RAIDController

RAIDControllerBlock 1Block 1 Block 1Block 1Block 1Block 1Block 0Block 0 Block 0Block 0Block 0Block 0

Host

RAID 1 ADVANTAGESHigh data availability and high I/O rate (small block size). Improves read performance - twice the read transaction rate of single disks, same write transaction rate as single disks100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement diskSimplest RAID storage subsystem design – easy to maintain

RAID 1 DISADVANTAGES

Expensive due to the extra capacity required to duplicate data. Overhead cost equals 100%, while usable storage capacity is 50%.May not support hot swap of failed disk when implemented with software. Use hardware implementation.

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RAID 0+1 – STRIPING AND MIRRORING

RAIDController

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

Host

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RAID 1+0 – MIRRORING AND STRIPING

RAIDController

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

Host

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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 REDUNDANCY: PARITY

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Parity Disk

0

84

1

95

2

106

3

117

0 1 2 3

8 9 10 114 5 6 7

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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

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RAID 3 – PARALLEL TRANSFER WITH DEDICATED PARITY DISK

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

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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

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RAID 5 – INDEPENDENT DISKS WITH DISTRIBUTED PARITY

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

Host

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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

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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

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HOT SPARES

RAIDController

RAIDController

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HOT SWAP

RAIDController

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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 a hot spare?