RAID levels

RAID 0:

RAID Level 0 requires a minimum of 2 drives to implement

Charistics and Advantages:

RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive

I/O performance is greatly improved by spreading the I/O load across many channels and drives

Best performance is achieved when data is striped across multiple 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 array being lost

Should never be used in mission critical environments
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RAID 1:

For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.

RAID Level 1 requires a minimum of 2 drives to implement

Charistics and Advantages:

One Write or two Reads possible per mirrored pair

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

Transfer rate per block is equal to that of a single disk

Under certain circumstances, RAID 1 can sustain multiple simultaneous 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, loading the 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"

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RAID 2 :

Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.

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 smaller word sizes – inefficient

Entry level cost very high – requires very high transfer rate requirement to justify

Transaction rate is equal to that of a single disk at best (with spindle synchronization)

No commercial implementations exist / not commercially viable

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RAID 3

The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads.

RAID Level 3 requires a minimum of 3 drives to implement

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
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RAID 4

Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads.

RAID Level 4 requires a minimum of 3 drives to implement

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
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RAID 5:

Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads.

RAID Level 5 requires a minimum of 3 drives to implement

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
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RAID 6:

Two independent parity computations must be used in order to provide protection against double disk failure. Two different algorithms are employed to achieve this purpose.

RAID Level 6 requires a minimum of 4 drives to implement

Characteristics & Advantages:

RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (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 sustain multiple simultaneous drive failures

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
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RAID 10:

RAID Level 10 requires a minimum of 4 drives to implement

Characteristics & Advantages:

RAID 10 is implemented as a striped array whose segments are RAID 1 arrays

RAID 10 has the same fault tolerance as RAID level 1

RAID 10 has the same overhead for fault-tolerance as mirroring alone

High I/O rates are achieved by striping RAID 1 segments

Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures

Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost

Disadvantages:

Very expensive / High overhead

All drives must move in parallel to proper track lowering sustained performance

Very limited scalability at a very high inherent cost
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RAID 50:

RAID Level 50 requires a minimum of 6 drives to implement

Characteristics & Advantages:

RAID 50 should have been called "RAID 03" because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays (during mid-90s)

RAID 50 is more fault tolerant than RAID 5 but has twice the parity overhead

High data transfer rates are achieved thanks to its RAID 5 array segments

High I/O rates for small requests are achieved thanks to its RAID 0 striping

Maybe a good solution for sites who would have otherwise gone with RAID 5 but need some additional performance boost

Disadvantages:

Very expensive to implement

All disk spindles must be synchronized, which limits the choice of drives

Failure of two drives in one of the RAID 5 segments renders the whole array unusable

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RAID 0+1:

RAID Level 0+1 requires a minimum of 4 drives to implement

Characteristics & Advantages:

RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays

RAID 0+1 has the same fault tolerance as RAID level 5

RAID 0+1 has the same overhead for fault-tolerance as mirroring alone

High I/O rates are achieved thanks to multiple stripe segments

Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability

Disadvantages:

RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array

Very expensive / High overhead

All drives must move in parallel to proper track lowering sustained performance

Very limited scalability at a very high inherent cost

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