RAID (Redundant Array of Independent Disks)

RAID stands for Redundant Arrayof Inexpensive (or sometimes “Independent”) Disks. RAID isa method of combining multiple hard disks in a single logical unit tooffer high availability, performance or a combination of both. Thisprovides better resilience and performance than a single disk drive.

Benefits of RAID

• Provides real-time data recovery with uninterrupted access when a hard drive fails
• Increases system uptime and network availability
• Protects against data loss
• Multiple drives working in parallel increases system performance

Software RAID

Many operating systems provide functionality for implementing software based RAID systems.  Thesoftware RAID systems generate the RAID algorithms using the server CPU,this can severely limit the RAID performance. Should a server failthe whole RAID system is lost. Cheap to implement and only need asingle SCSI controller.

Hardware RAID

All RAID algorithms are generated on the RAID controller board, thus freeing the server CPU.  Allows fullbenefits and data protection of RAID. More robust and fault tolerantthan software RAID. Requires dedicated RAID controller to work.

 RAID levels

Various RAID levels exist these are 0, 1, 2, 3, 4, 5, 6, 7, 10 (1+0), & 0+1.

The levels of RAID protectionvaries with the RAID level selected RAID levels 0 & 1 are nottechnically RAID as they have no redundancy in the event of drivefailure. The most common RAID levels are shown below.

RAID levels 2, 4, 6, 7 & 0+1 are a combination of the other RAID levels shown.

RAID – O – Striped Disk Array without Fault Tolerance:

Offers no redundancy or fault tolerance, hence does not truly fit the”RAID” acronym. In level 0, data is striped across drives,resulting in higher data throughput. Since no redundant information isstored, performance is very good, but the failure of any disk in thearray results in data loss. This level is commonly referred to asstriping.

Advantages No parity generation Easy to implement Cost effective Utilises full disk capacity

Disadvantages Not a true RAID No redundancy / fault tolerance Drive failure will result in data loss Not suitable in mission critical environments

  

 Dual controller Raid 0 offers better performance than a single controller

 RAID 1  – Mirroring & Duplexing :

Provides redundancy by writing all data to two or more drives.The performance of a level 1 array tends to be faster on reads andslower on writes compared to a single drive, but if either drive fails,no data is lost. This is a good entry-level redundant system, since onlytwo drives are required; however, since one drive is used to store aduplicate of the data, the cost per megabyte is high. This level iscommonly referred to as mirroring.

Advantages No parity generation Easy to implement Extremely fault tolerant Utilises full disk capacity 2 drives minimum

Disadvantages Inefficient use of disk space High disk overhead Doubles number of writes

RAID 3 – Parallel Transfer with Indipendent Parity :

Provides redundancy by writing all data to three or moredrives. This RAID 3 disk array provides excellent storage for videoimaging, streaming, publishing applications or any system that requireslarge file block transfers.

Advantages Single dedicated parity disk High read data rate High write data rate 4 drives minimum No performance degradation if drive fails Best and worst case performance similar

Disadvantages Inefficient with small file transfer

RAID 5 – Independent Data Disks with Distributed Parity Blocks (Distributed parity) :

Provides redundancy by writing data and parity informationacross three or more drives, thus increasing performance.

The RAID Level 5 provides the best combination of disk array technology. The most popular RAID level supplied.

Advantages Most flexible of all disk arrays Best balance cost / performance / protection of any RAID system Allows multiple simultaneous writes High read data rate Medium write data rate 3 drives minimum Ideal for small write applications Highly efficient

Disadvantages Inefficient with large file transfer Disk failure has an impact on performance

 
RAID 1 + 0 – Very High Reliability Combined with High Performance :

Provides very high performance and redundancy. Datais simultaneously mirrored and striped.

Can under circumstancessupport multiple drive failures.

Advantages Highly fault tolerant High data availability Very good read / write performance

Disadvantages Very expensive Drive spindles must be synchronised Not very scaleable

RAID 0+1 – High Data Transfer Performance :

Provides redundancy by writing all data to four or more drives.RAID 0+1 combines the benefits of RAID level 0 and RAID level 1.

This offers both striping and mirroring with no parity generation.

The RAID 0+1 also provides high performance and resilience to RAID 5.

Advantages No parity generation Easy to implement Utilises full disk capacity 4 drives minimum Higher performance than RAID 5

Disadvantages Inefficient use of disk space High disk overhead / Expensive Costly to deploy