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Topic: RAID Levels – 0,1,2,3,4,5,6
RAID
Redundant Arrays Of Independent Disks
Content:
What is RAID?
History behind RAID
Taxonomy of RAID levels
RAID : Level 0 to Level 6 What that level means & does? Working with help of diagram Characteristics & Advantages Disadvantages Recommended Applications
Benefits of using RAID Level
References
What is RAID?
It is multiple-disk database design.
It is a category of disk drives (two or more) in combination for fault tolerance and performance.
It has seven levels – zero to six
RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.
RAID allows you to store the same data redundantly (in multiple paces) in a balanced way to improve overall storage performance.
History of RAID:
In 1987, Patterson, Gibson and Katz at the University of California at Berkeley, published a paper entitled “A Case for Redundant Arrays of Inexpensive Disks (RAID)”
I/O becoming a performance bottleneck
Different RAID Levels:
Different levels which provides a different balance between performance, capacity and tolerance.
Following are they:
LEVEL 2
LEVEL 3
LEVEL 0
LEVEL 1
LEVEL 4
LEVEL 6
LEVEL 5
Striped Disk Array without FaultTolerance
Mirroring and Duplexing
Error-Correcting Coding
Bit-Interleaved Parity
Dedicated Parity Drive
Block Interleaved Distributed Parity
Independent Data Disks with Double Parity
LEVEL 0: Striped Disk Array without Fault Tolerance-
Minimum nos. of drives required : 2
Striping is the segmentation of logically sequential data, such as a single file, so that segments can be assigned to multiple physical devices
The first byte of the file is sent to the first drive, then the second to second drive and so on.
Stripes data across multiple disks without any redundant information.
Striping reduces the level of data availability since a disk failure will cause the entire array to be inaccessible.
Characteristics & Advantages:
RAID 0 implements a striped disk array, the data is brokendown into blocks and each block is written to a separate disk drive.
I/O performance is greatly improved by spreading the I/O loadacross many channels and drives.
Best performance is achieved when data is striped acrossmultiple controllers with only one drive per controller.
No parity calculation overhead is involved
Very simple design
Easy to implement
Disadvantages:
Not a "True" RAID because it is NOT fault-tolerant
The failure of just one drive will result in all data in an arraybeing lost
Should never be used in mission critical environments
Recommended Applications:
Video Production and Editing
Image Editing
Pre-Press Applications
Any application requiring high bandwidth
LEVEL 1: Mirroring and Duplexing -
Minimum nos. of drives required : 2
RAID-1 provides data redundancy.
Data written to one disk drive is simultaneously written to another disk drive, called the mirroring.
RAID-1 provides high data availability
In addition, read performance may be enhanced if the array controller allows simultaneous reads from both members of a mirrored pair.
Higher availability will be achieved if both disks in a mirror pair are on separate I/O busses, known as duplexing.
Characteristics & Advantages:
One Write or two Reads possible per mirrored pair
Twice the Read transaction rate of single disks, same Writetransaction rate as single disks.
100% redundancy of data means no rebuild is necessary in caseof a disk failure, just a copy to the replacement disk
Transfer rate per block is equal to that of a single disk
Under certain circumstances, RAID 1 can sustain multiplesimultaneous drive failures
Simplest RAID storage subsystem design
Disadvantages:
Highest disk overhead of all RAID types (100%) – inefficient
Typically the RAID function is done by system software, loadingthe CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended
May not support hot swap of failed disk when implemented in "software"
Recommended Applications:
Accounting
Payroll
Financial
Any application requiring very high availability
LEVEL 2: Error-Correcting Coding -
It is a theoretical entity.
It stripes data at bit level across an array of disks, then writes check bytes to other disks in the array.
The check bytes are calculated using a Hamming code.
Theoretical performance is very high, but it would be so expensive to implement that no-one uses it.
Characteristics & Advantages:
On the fly" data error correction
Extremely high data transfer rates possible
The higher the data transfer rate required, the better the ratio of data disks to ECC disks
Relatively simple controller design compared to RAID levels 3,4 & 5
Disadvantages:
Very high ratio of ECC disks to data disks with smallerword sizes – inefficient
Entry level cost vey high - requires very high transfer raterequirement to justify
Transaction rate is equal to that of a single disk at best(with spindle synchronization)
No commercial implementations exist / not commercially viable
LEVEL 3: Bit-Interleaved Parity -
Minimum nos. of drives required : 3
A block of data is striped over an array of disks, then parity data is written to a dedicated parity disk.
Successful implementations usually require that all the disks have synchronized rotation.
RAID3 is very effective for large sequential data, such as satellite imagery and video.
Characteristics & Advantages:
Very high Read data transfer rate
Very high Write data transfer rate
Disk failure has an insignificant impact on throughput
Low ratio of ECC (Parity) disks to data disks means high efficiency
Disadvantages:
Transaction rate equal to that of a single disk drive at best (if spindles are synchronized)
Controller design is fairly complex
Very difficult and resource intensive to do as a "software" RAID
Recommended Applications:
Video Production and live streaming
Image Editing
Video Editing
Prepress Applications
Any application requiring high throughput
LEVEL 4: Dedicated Parity Drive -
Minimum nos. of drives required : 3
Level 4 provides block-level striping (like Level 0) with a parity disk.
If a data disk fails, the parity data is used to create a replacement disk.
Characteristics & Advantages:
Very high Read data transaction rate
Low ratio of ECC (Parity) disks to data disks means high efficiency
High aggregate Read transfer rate
Disadvantages:
Quite complex controller design
Worst Write transaction rate and Write aggregate transfer rate
Difficult and inefficient data rebuild in the event of disk failure
Block Read transfer rate equal to that of a single disk
Recommended Applications:
Video Production and Editing
Image Editing
Pre-Press Applications
Any application requiring high bandwidth
LEVEL 5: Block Interleaved Distributed Parity -
Minimum nos. of drives required : 3
It distributes parity along with the data and requires that all drives but one be present to operate. The array is not destroyed by a single drive failure.
On drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user.
Example:
Start with four drives
Create an array using three of the physical drives, leaving the fourth as a hot-spare drive.
Then create a logical drive within that array.
The data is striped across the drives, creating blocks.
Notice that the storage of the data parity (denoted by *) also is striped, and it shifts from drive to drive.
A parity block ( *) contains a representation of the data from the other blocks in the same stripe. For example, the parity block in the first stripe contains data representation of blocks 1 and 2.
If a physical drive fails in the array, the data from the failed physical drive is reconstructed onto the hot-spare drive.
Characteristics & Advantages:
Highest Read data transaction rate
Medium Write data transaction rate
Low ratio of ECC (Parity) disks to data disks means high efficiency
Good aggregate transfer rate
Disadvantages:
Disk failure has a medium impact on throughput
Most complex controller design
Difficult to rebuild in the event of a disk failure (as compared to RAID level 1)
Individual block data transfer rate same as single disk
Recommended Applications:
File and Application servers
Database servers
Web, E-mail, and News servers
Intranet servers
Most versatile RAID level
LEVEL 6: Independent Data Disks with Double Parity -
Minimum nos. of drives required : 2
RAID 6 provides fault tolerance up to two failed drives.
This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive.
Example:
Start with six physical drives.
Create a logical drive using four physical drives, leaving two for hot spare drives.
The data is striped across the drives, creating blocks in the logical drive. The storage of the data parity (denoted by * and **) is striped, and it shifts from drive to drive as it does in RAID level-5.
If a physical drive fails in the array, the logical drive is degraded but remains fault tolerant.
If a second physical drive fails in the array, the data from the failed drives are reconstructed onto the hot-spare drives, and the data for the logical drive return to the original striping scheme.
Characteristics & Advantages:
RAID 6 is essentially an extension of RAID level 5 which allows foradditional fault tolerance by using a second independent distributed parityscheme (dual parity) Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives;RAID 6 provides for an extremely high data fault tolerance and can sustainmultiple simultaneous drive failures.
RAID 6 protects against multiple bad block failures while non-degraded
RAID 6 protects against a single bad block failure while operating in adegraded mode Perfect solution for mission critical applications
Disadvantages:
More complex controller design
Controller overhead to compute parity addresses is extremely high
Write performance can be brought on par with RAID Level 5 by using a custom ASIC for computing Reed-Solomon parity
Requires N+2 drives to implement because of dual parity scheme
Recommended Applications:
File and Application servers
Database servers
Web, E-mail, and News servers
Intranet servers
Excellent fault-tolerance with the lowest overhead
Table: RAID Levels
Benefits of RAID:
Data loss can be very dangerous for an organization
RAID technology prevents data loss due to disk failure
RAID technology can be implemented in hardware or software
Servers make use of RAID technology
Conclusion:
RAID offers a cost effective alternative to SLED through the use of data striping mirroring parity
Different RAID “levels” can be chosen to suit different functions for the computer.
Reference:
http://www.webopedia.com/TERM/R/RAID.html
http://www.acnc.com/raid
http://www.lascon.co.uk/hwd-raid.php
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