What is a disk array

A disk array (discarray) is composed of many disk drives or optical disk drives according to certain rules, such as striping, declustering, interleaving, etc., to form a fast, large-capacity external memory subsystem . Under the control and management of the array controller, it realizes fast, parallel or interleaved access, and has strong fault tolerance. From the user's point of view, although the disk array is composed of several, dozens, or even hundreds of disks, it can still be considered as a single disk, and its capacity can be as high as hundreds to gigabytes, so this technology Widely welcomed by multimedia systems.

The full name of the disk array is: redundanarrayofinexpensivedisk, referred to as raid technology. It is a disk redundancy technology proposed by Professor davidpatterson of the University of California at Berkeley in 1988. Since then, disk array technology has developed rapidly and has gradually matured. The following eight series have been basically recognized now.

1. raid0 (level 0 disk array)

raid0 is also known as data partitioning, that is, the data is distributed on multiple disks without fault tolerance measures. Its capacity and data transmission rate are n times the capacity of a single machine, n is the total number of disk units constituting the disk array, i / o transmission rate is high, but the average failure time mttf (meantimetofailure) is only one-nth of a single disk unit Therefore, the reliability of the zero-level disk array is the worst.

2.raid1 (level 1 disk array)

RAID1, also known as mirror disk, uses mirror fault tolerance to improve reliability. That is, each working disk has a mirror disk. Each time the data is written, the mirror disk must be written at the same time. When reading the data, only the work disk is read out. Once the working disk fails, it is immediately transferred to the mirror disk, the data is read from the mirror disk, and then the correct data of the working disk is restored by the system. Therefore, data can be reconstructed in this way, but the working disk and the mirror disk must maintain a one-to-one correspondence. This disk array has high reliability, but its effective capacity is reduced to less than half of the total capacity. Therefore, raid1 is often used in applications that have strict requirements on error rates, such as finance and finance.

3.raid2 (2 level disk array)

raid2 is also known as bit cross. It uses Hamming code for disk error checking, without crc (cyclic redundancy check) checking after each sector. The Hamming code is a (n, k) linear block code, where n is the length of the codeword, k is the number of data bits, and r is the number of bits used for inspection, so there are: n = 2r-1r = n-k

Therefore, bit-by-bit interleaving is most useful for Hamming code verification. This disk is suitable for reading and writing big data. But the overhead of redundant information is still too large, which prevents the widespread use of such disks.

4, raid3 (3-level disk array)

RAID3 is a single-disk fault-tolerant parallel transmission array disk. Its feature is to reduce the inspection disc to one (multiple raid2 verification discs, 1: 1 to daid1 verification discs), and the data is stored in each disc in bits or bytes (distributed and recorded in the same sector number within the group On each disk drive). Its advantage is that the bandwidth of the entire array can be fully utilized, which reduces the batch data transmission time; its disadvantage is that each read and write needs to affect the entire group, and only one i / o can be completed at a time.

5, raid4 (4-level disk array)

RAID4 is an array that can independently read and write to each disk in the group. There is only one check disk.
The difference between raid4 and raid3 is: raid3 is interleaved by bit or by byte, and raid4 is accessed by block (sector), you can operate on a certain disk separately, it does not need to be like raid3, even if every A small i / o operation also involves the entire group, and only involves two disk drives in the group (one data disk, one inspection disk). Thereby increasing the i / o rate of small amounts of data.

6, raid5 (5-level disk array)

RAID5 is a rotating parity independent access array. It is different from raid1, 2, 3, and 4 disk arrays in that it does not have a fixed check disk, but distributes its redundant parity information evenly on all disks to which the array belongs according to a certain rule. . Therefore, there are both data information and check information on the same disk drive. This change solves the problem of contention check disks, so daid5 allows multiple write operations in the same group concurrently. Therefore, raid5 is suitable for operations with large amounts of data, as well as various transactions. It is a fast, large-capacity and reasonably distributed disk array.

7, raid6 (6-level disk array)

RAID6 is a kind of dual-access even-access independent disk array. Its redundant detection and error correction information is evenly distributed on all disks, while the data is still stored in various disks in a cross-sectional manner in variable-size blocks. This type of disk array can tolerate double disk errors.

8, raid7 (7-level disk array)

RAID7 is based on RAID6 and uses cache technology, which greatly improves the transmission rate and response speed. The cache is a kind of cache memory, that is, data is written into the cache before being written to the disk array. Generally, the cache block size is the same as the data block size in the disk array, that is, one cache block corresponds to one disk block. Write data to two separate caches when writing, so that even if one of the caches fails, the data will not be lost. The write operation will respond directly at the cache level, and then go to the disk array. When data is written from the cache to the disk array, the data of the same track will be completed in one operation, which avoids the problem of writing many blocks of data multiple times and improves the speed. When reading, the host also reads directly from the cache, instead of reading from the array disk, reducing the number of read operations with the disk, which makes full use of the disk bandwidth.