Raid

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.

Diagram:

http://www.acnc.com/raidedu/0

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.

Diagram:

http://www.acnc.com/raidedu/1

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.

Diagram:

http://www.acnc.com/raidedu/3

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.

Diagram:

http://www.acnc.com/raidedu/4

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

Question & Answer