RAID (Redundant Array of Inexpensive Disks) 是什麼???

RAID (Redundant Array of Inexpensive Disks) 暫時有 RAID Level 0,1,2,3,4,5,6,10,50 及 0+1 這幾個不同的格式.

RAID Level 0 Striped Disk Array Without Fault Tolerance:
RAID Level 0 requires a minimum of 2 drives to implement

Characteristics & 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.

Recommended Applications
Video Production and Editing, Image Editing, Pre-Press Applications,Any application requiring high bandwidth.

RAID Level 1 Mirroring & Duplexing:
RAID Level 1 requires a minimum of 2 drives to implement, For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.

Characteristics & 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".

Recommended Applications
Accounting, Payroll, Financial, Any application requiring very high availability.

RAID Level 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.

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

RAID Level 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

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.

RAID Level 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

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.

RAID Level 5 Independent Data Disks with Distributed Parity Disk:
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

Recommended Applications
File and Application servers, Database servers, Web, E-mail, and News servers, Intranet servers, Most versatile RAID level.

RAID Level 6 Independent Data Disks with Two Independent Distributed Parity Schemes:

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 (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.

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 two-dimensional parity scheme.

RAID Level 10 Very High Reliability Combined with High Performance:
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.

Recommended Applications
Database server requiring high performance and fault tolerance

RAID Level 50 High I/O Rates & Data Transfer Performance:
RAID Level 50 requires a minimum of 5 drives to implement

Characteristics & Advantages
RAID 50 should really be called "RAID 03" because it is implemented as a striped (RAID level 0) array whose segments are RAID 3 arrays, RAID 50 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, 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 3 but need some additional performance boost.

Disadvantages
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 Level 0+1 High Data Transfer Performance:
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.

Recommended Applications
Imaging applications, General fileserver.