RAID ARRAYSRedundant 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

RA

ID A

rrays

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).

RA

ID A

rraysRAID - REDUNDANT ARRAY OF INDEPENDENT DISKS

RAIDController

RAID Array

Host

RA

ID A

rrays

RAID COMPONENTS

RAIDController

Logical Array

Logical Array

Physical Array

RAID Array

Host

RA

ID A

rraysDATA ORGANIZATION: STRIPS AND STRIPES

Stripe 1Stripe 2Stripe 3

Strips

RA

ID A

rrays

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 0A 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

- 10

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

Host

ADVANTAGES OF RAID 0I/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 0Not 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

- 13

RAIDControllerBlock 1 Block 1Block 1Block 0 Block 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.

RA

ID A

rraysRAID 0+1 – STRIPING AND MIRRORING

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

Host

RA

ID A

rraysRAID 1+0 – MIRRORING AND STRIPING

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

Host

RA

ID A

rrays

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

RA

ID A

rrays

RAID REDUNDANCY: PARITY

- 19

Parity Disk

0

84

1

95

2

106

3

117

0 1 2 3

8 9 10 114 5 6 7

RAIDController

Host

RA

ID A

rrays

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

RA

ID A

rraysRAID 3 – PARALLEL TRANSFER WITH DEDICATED PARITY DISK

RAIDController

Block 1

Block 2

Block 3

P 0 1 2 3

Block 0Block 3Block 2Block 1Block 0

ParityGenerated

Host

RA

ID A

rraysRAID 4 – STRIPING WITH DEDICATED PARITY DISK

RAIDControllerP 0 1 2 3

Block 0Block 0

Block 0

Block 4

Block 1

Block 5

Block 2

Block 6

Block 3

Block 7

P 0 1 2 3

P 4 5 6 7

ParityGenerated

Block 0

P 0 1 2 3

Host

RA

ID A

rraysRAID 5 – INDEPENDENT DISKS WITH DISTRIBUTED PARITY

Block 0

P 0 1 2 3

Block 7

RAIDControllerP 0 1 2 3

Block 0Block 4Block 0

Block 1

Block 5

Block 2

Block 6

Block 3

ParityGenerated

Block 0

P 0 1 2 3

Block 4

P 4 5 6 7P 4 5 6 7

Block 4

P 4 5 6 7

Block 4ParityGenerated

Host

RA

ID A

rrays

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

RA

ID A

rrays

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

RA

ID A

rrays

HOT SPARES

RAIDController

RA

ID A

rrays

HOT SWAP

RAIDController

RAIDController

RAIDController

RA

ID A

rrays

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?