{"id":232,"date":"2020-03-06T14:21:30","date_gmt":"2020-03-06T12:21:30","guid":{"rendered":"https:\/\/tekmart.co.za\/t-blog\/?p=232"},"modified":"2020-03-06T14:32:42","modified_gmt":"2020-03-06T12:32:42","slug":"raid-redundant-array-of-independent-disks-a-tech-definition","status":"publish","type":"post","link":"https:\/\/tekmart.co.za\/t-blog\/raid-redundant-array-of-independent-disks-a-tech-definition\/","title":{"rendered":"RAID (redundant array of independent disks)-a tech definition"},"content":{"rendered":"Reading Time-approximately:<\/span> 8<\/span> minutes<\/span><\/span>\n

Posted by: <\/p>\n\n\n\n

\"\"\/<\/a><\/figure>\n\n\n\n

Margaret Rouse<\/strong><\/p>\n\n\n\n

WhatIs.com – TechTarget – WhatIs.com<\/a> <\/strong><\/p>\n\n\n\n

Contributor(s): Alexander Gillis<\/a>, Erin Sullivan, Brien Posey, Con Diamantis and Yoshinobu Yamamura <\/p>\n\n\n\n

RAID (redundant array of independent disks) is a way of storing the same data in different places on multiple hard disks<\/a>\n or solid-state drives to protect data in the case of a drive failure. \nThere are different RAID levels, however, and not all have the goal of \nproviding redundancy<\/a>.<\/p>\n\n\n\n

How RAID works<\/strong><\/h3>\n\n\n\n

RAID works by placing data on multiple disks and allowing input\/output (I\/O<\/a>)\n operations to overlap in a balanced way, improving performance. Because\n the use of multiple disks increases the mean time between failures (MTBF<\/a>), storing data redundantly also increases fault tolerance<\/a>.<\/p>\n\n\n\n

RAID arrays appear to the operating system (OS) as a single logical \ndrive. RAID employs the techniques of disk mirroring or disk striping. \nMirroring will copy identical data onto more than one drive. Striping partitions<\/a> helps spread data over multiple disk drives. Each drive’s storage space is divided into units ranging from a sector<\/a> (512 bytes<\/a>) up to several megabytes<\/a>. The stripes of all the disks are interleaved and addressed in order.<\/p>\n\n\n\n

\"RAID
\n Image of a five-tray RAID hard drive\n <\/figcaption><\/figure>\n\n\n\n

Disk mirroring and disk striping can also be combined in a RAID array.<\/p>\n\n\n\n

In a single-user system where large records<\/a>\n are stored, the stripes are typically set up to be small (perhaps 512 \nbytes) so that a single record spans all the disks and can be accessed \nquickly by reading all the disks at the same time.<\/p>\n\n\n\n

In a multi-user system, better performance requires a stripe wide \nenough to hold the typical or maximum size record, allowing overlapped \ndisk I\/O across drives.<\/p>\n\n\n\n

RAID controller<\/strong><\/h3>\n\n\n\n

A RAID controller<\/a>\n is a device used to manage hard disk drives in a storage array. It can \nbe used as a level of abstraction between the OS and the physical disks,\n presenting groups of disks as logical units. Using a RAID controller \ncan improve performance and help protect data in case of a crash.<\/p>\n\n\n\n

A RAID controller may be hardware- or software-based. In a hardware-based RAID<\/a> product, a physical controller manages the array. The controller can also be designed to support drive formats such as SATA<\/a> and SCSI<\/a>. A physical RAID controller can also be built into a server’s motherboard.<\/p>\n\n\n\n

With software-based RAID<\/a>,\n the controller uses the resources of the hardware system, such as the \ncentral processor and memory. While it performs the same functions as a \nhardware-based RAID controller, software-based RAID controllers may not \nenable as much of a performance boost and can affect the performance of \nother applications on the server.<\/p>\n\n\n\n

If a software-based RAID implementation isn’t compatible with a \nsystem’s boot-up process, and hardware-based RAID controllers are too \ncostly, firmware<\/a> or driver-based RAID is another potential option.<\/p>\n\n\n\n

Firmware-based RAID controller chips are located on the motherboard,\n and all operations are performed by the CPU, similar to software-based \nRAID. However, with firmware, the RAID system is only implemented at the\n beginning of the boot process. Once the OS has loaded, the controller \ndriver takes over RAID functionality. A firmware RAID controller isn’t \nas pricy as a hardware option, but it puts more strain on the computer’s\n CPU. Firmware-based RAID is also called hardware-assisted software \nRAID, hybrid model RAID and fake RAID.<\/p>\n\n\n\n

RAID levels<\/strong><\/h3>\n\n\n\n

Raid devices will make use of different versions, called levels. The\n original paper that coined the term and developed the RAID setup \nconcept defined six levels of RAID — 0 through 5. This numbered system \nenabled those in IT to differentiate RAID versions. The number of levels\n has since expanded and has been broken into three categories: standard,\n nested and nonstandard RAID levels.<\/p>\n\n\n\n

Standard RAID levels<\/strong><\/h3>\n\n\n\n

RAID 0<\/strong><\/a>.<\/strong>\n This configuration has striping, but no redundancy of data. It offers \nthe best performance, but it does not provide fault tolerance.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 1<\/strong><\/a>.<\/strong> Also known as disk mirroring<\/em>,\n this configuration consists of at least two drives that duplicate the \nstorage of data. There is no striping. Read performance is improved \nsince either disk can be read at the same time. Write performance is the\n same as for single disk storage.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 2<\/strong><\/a>.<\/strong> This configuration uses striping across disks, with some disks storing error checking and correcting (ECC<\/a>)\n information. RAID 2 also uses a dedicated Hamming code parity; a linear\n form of error correction code. RAID 2 has no advantage over RAID 3 and \nis no longer used.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 3<\/strong><\/a>.<\/strong> This technique uses striping and dedicates one drive to storing parity<\/a> information. The embedded ECC information is used to detect errors. Data recovery<\/a>\n is accomplished by calculating the exclusive information recorded on \nthe other drives. Since an I\/O operation addresses all the drives at the\n same time, RAID 3 cannot overlap I\/O. For this reason, RAID 3 is best \nfor single-user systems with long record applications.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 4<\/strong><\/a>.<\/strong>\n This level uses large stripes, which means a user can read records from\n any single drive. Overlapped I\/O can then be used for read operations. \nSince all write operations are required to update the parity drive, no \nI\/O overlapping is possible.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 5<\/strong><\/a>. <\/strong>This level is based on parity block<\/a>-level\n striping. The parity information is striped across each drive, enabling\n the array to function even if one drive were to fail. The array’s \narchitecture allows read and write operations to span multiple drives —\n resulting in performance better than that of a single drive, but not as\n high as that of a RAID 0 array. RAID 5 requires at least three disks, \nbut it is often recommended to use at least five disks for performance \nreasons.<\/p>\n\n\n\n

RAID 5 arrays are generally considered to be a poor choice for use \non write-intensive systems because of the performance impact associated \nwith writing parity data. When a disk fails, it can take a long time to \nrebuild a RAID 5 array.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n

RAID 6<\/strong><\/a>.<\/strong>\n This technique is similar to RAID 5, but it includes a second parity \nscheme distributed across the drives in the array. The use of additional\n parity enables the array to continue to function even if two disks fail\n simultaneously. However, this extra protection comes at a cost. RAID 6 \narrays often have slower write performance than RAID 5 arrays.<\/p>\n\n\n\n

\"RAID<\/figure>\n\n\n\n