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RAID-Definitions

RAID 0 : Striped Disk Array without Fault Tolerance

RAID 1 : Mirroring and Duplexing

RAID 2 : Hamming Code ECC

RAID 3 : Parallel transfer with parity

RAID 4 : Independent Data disks with shared Parity disk

RAID 5 : Independent Data disks with distributed parity blocks

RAID 6 : Independent Data disks with two independent distributed parity schemes

RAID 7 : Optimized Asynchrony for High I/O Rates as well as High Data Transfer Rates

RAID 10 : Very High Reliability combined with High Performance

RAID 53 : High I/O Rates and Data Transfer Performance

RAID 0+1 : High Data Transfer Performance





RAID 0 : Striped Disk Array without Fault Tolerance

RAID Level 0 requires a minimum of 2 drives to implement
AdvantagesDisadvantages
RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive

No parity calculation overhead is involved

Very simple design

Easy to implement
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



RAID 1 : Mirroring and Duplexing

RAID Level 1 requires a minimum of 2 drives to implement
AdvantagesDisadvantages
One Write or two Reads possible per mirrored pair

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
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.



RAID 2 : Hamming Code ECC

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.
AdvantagesDisadvantages
'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
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.

No commercial implementations exist.



RAID 3 : Parallel transfer with parity

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
AdvantagesDisadvantages
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
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



RAID 4 : Independent Data disks with shared Parity disk

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
AdvantagesDisadvantages
Very high Read data transaction rate

Low ratio of ECC (Parity) disks to data disks means high efficiency

High aggregate Read transfer rate

Low ratio of ECC (Parity) disks to data disks means high efficiency
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



RAID 5 : Independent Data disks with distributed parity blocks

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
AdvantagesDisadvantages
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
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



RAID 6 : Independent Data disks with two independent distributed parity schemes

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
AdvantagesDisadvantages
RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional 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
Very complex controller design

Controller overhead to compute parity addresses is extremely high

Very poor write performance

Requires N+2 drives to implement because of two-dimensional parity scheme



RAID 7 : Optimized Asynchrony for High I/O Rates as well as High Data Transfer Rates

Fully implemented process oriented real time operating system resident on embedded array control microprocessor.
RAID 7 is a registered trademark of Storage Computer Corporation.
AdvantagesDisadvantages
Overall write performance is 25% to 90% better than single spindle performance and 1.5 to 6 times better than other array levels

Host interfaces are scalable for connectivity or increased host transfer bandwidth

Small reads in multi user environment have very high cache hit rate resulting in near zero access times

No extra data transfers required for parity manipulation
One vendor proprietary solution

Extremely high cost per MB

Very short warranty

Not user serviceable

Power supply must be UPS to prevent loss of cache data



RAID 10 : Very High Reliability combined with High Performance

RAID Level 10 requires a minimum of 4 drives to implement
AdvantagesDisadvantages
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

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



RAID 53 : High I/O Rates and Data Transfer Performance

RAID Level 53 requires a minimum of 5 drives to implement
AdvantagesDisadvantages
RAID 53 should really be called 'RAID 03' because it is implemented as a striped (RAID level 0) array whose segments are RAID 3 arrays

RAID 53 has the same fault tolerance as RAID 3 as well as the same fault tolerance overhead

High data transfer rates are achieved thanks to its RAID 3 array segments

Maybe a good solution for sites who would have otherwise gone with RAID 3 but need some additional performance boost
Very expensive to implement

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

Byte striping results in poor utilization of formatted capacity



RAID 0+1 : High Data Transfer Performance

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

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

Very limited scalability at a very high inherent cost

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




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