What is RAID and What You Need To Know About It

Redundant Array of Inexpensive Disks (RAID) is a technology that enables storage of the same data in different locations, on multiple hard disks. The main aim of the technology is to improve computer performance. It is has been discovered that by placing data in multiple storage locations on separate disks, input/output can overlap increasing time between failures. This helps improve fault tolerance while increasing storage capacity. To an operating system, RAID appears as a single logical disk.

RAID uses Disk stripping, a technology that involves partitioning each of the storage disk drives into several units ranging from 512bytes to several MBs. The units (stripes) are typically interleaved and addressed in order. For single users with large records, the stripes are usually as small as 512 bytes. This way a single record spans the whole disk making searches quick and easy as all disks can be read at the same time. For multi-users systems however, stripes are made to be wide enough to hold a maximum size record. This improves performance and allows for overlapped input/output across all the drives.

Implementing RAID

To implement RAID, you will need a number of disk drives and either  hardware or a software controller. Software RAID runs on a server thus utilizing server resources. For this reason, it may cause other server processes to run slowly. Nevertheless, most server OSs have built in RAID management tools. One can also set up RAID by adding a RAID controller to the server or desktop PC. RAID controller runs just as normal software with the only difference being that it uses its own processor rather than the system CPU. However, some RAID controllers also come without a separate processor and thus run on the system CPU, these are called fake RAID.

Another way of implementing RAID is to purchase a pre-built RAID array from a vendor. The package usually comes with two RAID controllers and a group of disks- each in its own housing. Virtually every storage vendor sells products that support RAID. Some known vendors include Intel, NetApp, Dell, HP, Fujitsu and Media sonic. There are also several open source projects on RAID technology such as Salamander, SnapRaid, RaidEye and LinuxRAID.

Types of RAID

There are at least 9 types of RAID. Firstly, there is what is known as non-redundant RAID-0, which has no striping, no redundancy of data and it is the most stable form of RAID. RAID-1 provides the best multi-user performance. It has at least two drives duplicating the storage data and either disk can be accessed at the same time. Here, writing performance is the same as for single disk storage. This type also has the best fault tolerance. It is also known as disk mirroring.

Between RAID-2 and RAID-S (parity Raid) exists RAID-3. 4. 5. 6. 7 and (10. 50 or 5+0 and 53 or 5+3.). RAID-3 is the best for single users with a long record applications; RAID-5 is best for multi-users systems where performance is not critical or where only a few write operations are needed. RAID-10 combines RAID-1 and RAID-0 and can be implemented either as RAID 1+0 or as RAID 0+1. RAID-50 consists of a series of RAID-5 groups that are stripped in RAID-0 fashion. RAID-53 uses RAID-0 style stripping for RAID-3 virtual disk blocks.

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