Thursday, April 5, 2012

What is RAID?

RAID is the abbreviation of redundant Array of Independent Disk. Redundant Array of Disk technology is raised by University of California at Berkeley in 1987. With a simple explanation, numerous disks are combined by RAID Cnotroller into single virtual disk with big capacity. The adoption of RAID brings huge profits to memory system. Among them, improvement of transmission rate and support of fault-tolerant are most significant.

Brief Introduction
The unique feature of Redundant Array of Independent Disks is that reading speed of numerous is quicken and it provides Fault Tolerant. Therefore, RAID is not Back Solution but Storage in usual access data. 

In short, RAID is a technology that combines numerous disks (physical hard disk) into an array of disk (logical disk) in different ways and provides data backup technology and higher memory property than that of single hard disk. The different forms of disk array are called RAID Levels.

In disk array, different technologies are applied in different processes. This is called RAID Level (RAID is the abbreviation of redundant Array of Independent Disk), and every level stands for a technology. Now RAID 0~RAID 5 is the standard accepted by the industry. This "level" does not mean the level of technology. Therefore, Level 5 is not higher than Level 3 and Level1 is not lower than Level 4. As which RAID level should be adopted, it totally depends on the operating system and application.

A basic concept in RAID is EDAP (Extended Data Availability and Protection), which emphasizes on expandability and Fault Tolerant system. That's why it is appealing to many companies such as Mylex,IBM,HP,Compaq,Adaptec,Infortrend and so on. Besides, that the following operations without program halt can be realized is also important.

RAID is in support of automatic detection of fault hard disk
RAID is in support of rebuilding data of fault tracks in disk.
RAID is in support of Hot Spare
RAID is in support of Hot Swap
RAID is in support of disk capacity expansion

1. Improvement of storage capacity Capacity of numerous disks composes large memory space.
2. Reduction of unit capacity cost:
The price of 1 MB of maximum capacity disk is much higher than that of normal disk, so the price of normal disks array is much lower.

3. Development of storage speed
Improving speed of single disk is limited by technology in each period and it is difficult to futher it on. However, RAID can allocate data access to several disks, which improve the overall speed multiply.

4. Reliability
Two arrays of disks can be used to complete mirror storage, and this kind of security method is so important for net work server.  

5. Fault Tolerant
One key function of RAID controller is Fault Tolerant processing. If error occurs in single disk in arrays, the application of whole array is not influenced. And senior RAID controller can also save data. 6. Support for ATA/66/100 in IDE RAID RAID is also divided into 2 types: SCSI RAID and IDE RAID, and IDE RAID are much chipper. If the motherboard does not support ATA/66/100, by using RAID, it can be realized.

RAID brings huge profits to memory system (or server of built-in memory). Among them, improvement of transmission rate and support of fault-tolerant are most significant.

By applying several disks at the same, RAID improves transmission rate. And it increases data throughput of memory system by storing and reading data in several disks simultaneously. RAID can make several disks transmit data at the same time, and these disk drives are single disk logically. So the speed of RAID is several times even dozens of times as the speed of single disk. And this is the initial purpose of RAID. At that time, CPU speed improved quickly but data transfer rate of disk drive could not be increased greatly and this contradiction must be solved. RAID made it at last.

Through the data validation, RAID provides Fault Tolerant. If writing CRC code in disk is not considered, normal disk has no Fault Tolerant. That is another reason for application of RAID. The Fault Tolerant of RAID is based on the hardware fault tolerant of every disc drive, so it provides high security. In RAID, there are comparatively complete mutual calibration and recovery measures even directly mutual mirror backup, which increase the capacity of Fault Tolerant in RAID and the stability of system.

Types and Applications
Based on different framework, RAID can be divided into several types: software RAID, hardware RAID and External RAID.

Software RAID has been contained in system in many conditions and become one function of system such as Windows, Netware and Linux. All the operations in software RAID are completed by CPU, so the use ratio of system resources is very high, which degrade system performance. There is no need to add any hardware device in soft RAID, for it is provided with all ready-made resources by system, mainly from CPU.

Usually Hardware RAID is a PCI card with processor and internal storage. For the processor provides all required resources for RAID, it takes up no system resource,which improve system performance. One function of hardware RAID is to connect built-in hard disk, Hot Swap backplane or built-out storage device. No matter which kind of disk in connected, it is in the control of RAID card, which is, controlled by system. In the system, driver for hardware RAID PIC card is usually needed or the system will not support it. Disk array is generated before or after the installation of system and it is considered as a large hard disk that has Fault Tolerant and redundancy functions. Not only one ready-made system can be added into the disk array. It also supports capacity expansion. The process is very simple. Users only need to add a new hard disk and execute a few instructions and system can apply the added capacity at any time.

The built-out RAID is one kind of RAID. But RAID card is not installed in system but installed in built-out storage device that is connected to SCSI card in system. The system has no RAID function as it only has a SCSI card. Therefore, all RAID functions are transfered to the built-out storage device. Its advantage is that the built-out storage device can connect more hard disk and it is not influenced by the size of system crate. And some advanced technologies such as dual fault tolerant need several servers to connect a built-out storage device to provide Fault Tolerant function. One application of the built-out RAID is the installation of any operating system, so the operating system has on influence on it.there is only one SCSI card instead of RAID card in system. And the built-out RAID is not a special device but only a large hard disk of the system and the SCSI card. Thereby, any operating system can be installed in this built-out RAID. The only demand is that driver of the operating system should be installed in the SCSI card. 

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