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 andor replicate data among multiple ha ID: 379131
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RAID Arrays
Redundant Array of Inexpensive DiscsSlide2
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 drivesSlide3
Why Use RAID?
Improved Reliability
Improved Performance
Fault Tolerance
Improved Availability
Higher Data SecuritySlide4
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).
RAID ArraysSlide5
RAID - Redundant Array of Independent Disks
RAID Arrays
RAID
Controller
RAID Array
HostSlide6
RAID Components
RAID Arrays
RAID
Controller
Logical Array
Logical Array
Physical Array
RAID Array
HostSlide7
Data Organization: Strips and Stripes
RAID Arrays
Stripe 1
Stripe 2
Stripe 3
StripsSlide8
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 ArraysSlide9
RAID 0
A striped set of at least two disks without parity
The data is broken down into blocks and each block is written to a separate disk drive
Best performance is achieved when data is striped across multiple controllers with only one drive per controller Slide10
RAID 0 – Striped Array with no Fault Tolerance
RAID Arrays
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10
RAID
Controller
Block 4
Block 4
Block 3
Block 3
Block 2
Block 2
Block 1
Block 1
Block 0
Block 0
HostSlide11
Advantages of RAID 0
I/O performance is greatly improved by spreading the I/O load across many channels and drives
No parity calculation overhead is involved
Very simple design
Easy to implement Slide12
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 lost
Should never be used in mission critical environments Slide13
RAID 1 – Disk Mirroring
RAID Arrays
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13
RAID
Controller
Block 1
Block 1
Block 1
Block 0
Block 0
Block 0
HostSlide14
RAID 1 Advantages
High 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 disks
100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk
Simplest RAID storage subsystem design – easy to maintainSlide15
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.Slide16
RAID 0+1 – Striping and Mirroring
RAID Arrays
RAID
Controller
Block 3
Block 3
Block 3
Block 2
Block 2
Block 2
Block 1
Block 1
Block 1
Block 0
Block 0
Block 0
HostSlide17
RAID 1+0 – Mirroring and Striping
RAID Arrays
RAID
Controller
Block 3
Block 3
Block 3
Block 2
Block 2
Block 2
Block 1
Block 1
Block 1
Block 0
Block 0
Block 0
HostSlide18
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
RAID ArraysSlide19
RAID Redundancy: Parity
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19
RAID Arrays
Parity Disk
0
8
4
1
9
5
2
10
6
3
11
7
0 1 2 3
8 9 10 11
4 5 6 7
RAID
Controller
HostSlide20
Parity Calculation
RAID Arrays
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 ArraySlide21
RAID 3 – Parallel Transfer with Dedicated Parity Disk
RAID Arrays
RAID
Controller
Block 1
Block 2
Block 3
P 0 1 2 3
Block 0
Block 3
Block 2
Block 1
Block 0
Parity
Generated
HostSlide22
RAID 4 – Striping with Dedicated Parity Disk
RAID Arrays
RAID
Controller
P 0 1 2 3
Block 0
Block 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
Parity
Generated
Block 0
P 0 1 2 3
HostSlide23
RAID 5 – Independent Disks with Distributed Parity
RAID Arrays
Block 0
P 0 1 2 3
Block 7
RAID
Controller
P 0 1 2 3
Block 0
Block 4
Block 0
Block 1
Block 5
Block 2
Block 6
Block 3
Parity
Generated
Block 0
P 0 1 2 3
Block 4
P 4 5 6 7
P 4 5 6 7
Block 4
P 4 5 6 7
Block 4
Parity
Generated
HostSlide24
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 ArraysSlide25
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 serverProvides more flexibility for hardware, which can reduce the costPerformance is dependent on CPU loadHas limited functionality
RAID ArraysSlide26
Hot Spares
RAID Arrays
RAID
ControllerSlide27
Hot Swap
RAID Arrays
RAID
Controller
RAID
Controller
RAID
ControllerSlide28
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?
RAID Arrays