Overview of RAID

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RAID is the abbreviation for "Redundant array of independent disk", which is an independent redundant array of disks. Redundant disk array technology was born in 1987 by the University of California, Berkeley.

RAID disk array (redundant array of independent disks)
Simply explained, that is, the N-disk drive through the raid Controller (Hardware,software) combined into a virtual single large-capacity hard disk, which is characterized by the speed of n-drive reading faster and provide fault-tolerant Fault tolerant, So RAID is storage not backup Solution as the main access data.

RAID 0: Stripe group without error control

To implement RAID0 must have more than two hard drives, RAID0 implements the stripe group, the data is not stored on a hard disk, but is divided into blocks of data stored on different drives. Because the data is distributed across different drives, the data throughput is greatly increased and the load on the drive is more balanced. If exactly the data you need is the most efficient on a different drive. It does not need to compute the checksum code, the realization is easy. Its disadvantage is that it does not have data error control, if the data in a drive is wrong, even if the data on the other disk is correct and useless. It should not be used for situations where data stability is required. It is more appropriate for users to use RAID0 for images (including animation) editing and other occasions where transmission is more demanding. At the same time, RAID can increase the data transfer rate, such as the required read files distributed on two hard drives, the two hard drives can be read at the same time. The original time to read the same file was shortened to 1/2. At all levels, RAID 0 is the fastest speed. But RAID 0 has no redundancy, and if one disk (physical) is corrupted, all of the data is unusable.
　　RAID 1: Mirror image structure
For devices using this RAID1 structure, the RAID controller must be able to read two disks at the same time and write to two mirror disks. You can also see from the following structure diagram that you must have two drives. Because it is the mirror structure in a set of disk problems, you can use the mirror image, improve the system fault-tolerant ability. It is relatively easy to design and implement. The disk can read only one piece of data per read, which means that the data block transfer rate is the same as that of the individual disk. Because the RAID1 is very complete, so it has a great impact on the processing power of the system, the usual RAID function is implemented by the software, and this implementation method can greatly affect the server efficiency when the server load is heavy. When your system requires very high reliability, such as data statistics, then use RAID1 is more appropriate. and RAID1 technology to support the "hot swap", that is, the continuous power of the case of the failure of the disk to replace the replacement, as long as the recovery from the mirror disk data can be. When the primary hard drive is damaged, the mirrored hard drive will work instead of the main hard drive. The mirrored hard disk is the equivalent of a backup disk, and it is conceivable that the security of this hard disk mode is very high and that RAID 1 data security is best for all RAID levels. But its disk utilization is only 50%, the lowest of all RAID levels.

RAID5: Distributed parity independent disk architecture RAID5

From its schematic, it can be seen that its parity code exists on all disks, where the p0 represents the parity value of the No. 0 band, and the other meanings are the same. The efficiency of RAID5 is very high, the writing efficiency is general, and the block-type collective access efficiency is good. Because parity codes are on different disks, they improve reliability and allow single disk errors. RAID 5 is also a data parity to ensure data security, but it is not a separate hard disk to store data parity bit, but to the data section of the check bit interaction on each hard disk. In this way, any hard drive that is damaged can reconstruct the corrupted data based on the parity bit on the other hard disk. The utilization of the hard disk is n-1. But it is not good for the parallelism of data transmission, and the design of controller is very difficult. The important difference between RAID 3 and RAID 5 is that RAID 3 involves all the array disks for each data transfer. For RAID 5, most data transfers are performed on a single disk and can be done in parallel. In RAID 5, there is "write loss", that is, each write operation, will produce four actual read/write operations, of which two read the old data and parity information, two times to write new data and parity information. The advantage of RAID-5 is that it provides redundancy (still running after a disk drop), higher disk space utilization (n-1/n), faster read and write (N-1 times). But when the disk is off, the efficiency of the operation is greatly reduced.

RAID10: High reliability and efficient disk structure

This structure is nothing more than a band structure plus a mirror structure, because the two structures have advantages and disadvantages, so they can complement each other, to achieve both high efficiency and high speed can also be. You can combine the advantages and disadvantages of both structures to understand this new structure. The price of this new structure is high, the scalability is not good. It is mainly used in databases with small capacity but requiring speed and error control.