About RAID
Information Technology, Storage January 31st, 2008RAID Level 0: Striped Disk Array without Fault Tolerance
RAID Level 0 requires a minimum od 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
Characteristics & Advantages
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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
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 Blocks
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 (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
- RAID 6 protects against multiple bad block failures while non-degraded
- RAID 6 prodects against a single bad block failure while operating in a degraded 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 and E-mail servers Intranet servers Excellent fault-tolerance with the lowest overhead
RAID Level 10: Very High Reliability combined with High Performance
RAID Level 10 requires a minimum of 4 drives to implementCharacteristics & 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 6 drives to implement 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) Most current RAID 50 implementation is illustrated above 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 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 unusableRAID Level 0+1: High Data Transfer Performance
RAID Level 0+1 requires a minimum of 4 drives to implementCharacteristics & 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
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