A brief introduction to RAID (Redundant Arrays of Independent Disks)

       RAID，Redundant Arrays of Independent Disks，which is an array of independent disks with redundancy capabilities.

       Disk arrays are made up of many less expensive disks that are combined into a large capacity disk group, using the additive effect of individual disks providing data to enhance the performance of the entire disk system. Using this technology, data can be cut into many segments and stored on individual hard disks. Disk arrays are connected as stand-alone systems either directly outside the host or through a network to the host, and the disk array has multiple ports that can be connected by different hosts or different ports. A host connecting different ports of the array can increase the transfer speed.

The style of disk arrays can be divided into three kinds:

1. External disk array cabinet. External disk array cabinet is most often used on large servers, with hot swap (Hot Swap) characteristics, the price of these products are very expensive; 

2. Internal disk array card. Internal disk array card, the price is relatively cheap, but requires a high degree of installation skills, suitable for technical personnel to use the operation;

3. The use of software to emulate. The use of software emulation means that the disk management function provided by the network operating system itself will be connected to the ordinary SCSI card on multiple hard disks configured as logical disks to form an array.

Advantages of disk arrays:

1. Increasing the transfer rate;

2. Provide fault tolerance through data verification.

Disadvantages of disk arrays RAID 0 and RAID 1:

1. RAID 0 has no redundancy; if one disk is (physically) damaged, all data is unusable;

2. RAID 1 disk utilization is only up to 50% (with two disks), which is the lowest of all RAID levels;

3. RAID 0+1 can provide data security for the system, but the level of security is lower than Mirror while the disk space utilization is higher than Mirror.