The difference between disk array and hard disk

The difference between disk array and hard disk

A disk array (DiscArray) consists of a number of disk units or CD players, such as striping, decibeling, interleaving, etc. . It is fast, parallel or interleaved under the control and management of the array controller, 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 several hundred to thousands of gigabytes. Therefore, this technology Widely welcomed by multimedia systems.

The following eight series are now generally recognized.

1. RAID0 (0-level disk array)

RAID0, also known as data partitioning, distributes data across multiple disks without fault tolerance. Its capacity and data transfer rate are N times that of the stand-alone capacity. N is the total number of disk units that make up the disk array. The I/O transfer rate is high, but the MTBF (MeanTimeToFailure) is only one-N of the single disk drive. Therefore, the reliability of the zero-order disk array is the worst.

2. RAID1 (level 1 disk array)

RAID1, also known as a mirrored disk, uses mirrored fault tolerance to improve reliability. That is, each work disk has a mirror disk. Each time data is written, it must be written to the mirror disk at the same time. When reading data, it is only read from the work disk. Once the work 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 work disk is restored by the system. Therefore, the data can be reconstructed in this way, but the work disk and the mirror disk must maintain a one-to-one correspondence. This disk array is highly reliable, but its effective capacity is reduced to less than half of the total capacity. Therefore, RAID1 is often used in applications where the error rate is extremely strict, such as finance and finance.

3. RAID2 (2 level disk array)

RAID 2, also known as bit crossing, uses Hamming code for disc error checking, eliminating the need for a CRC (CyclicReDundancycheck) check 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 bits of the data, and r is the number of bits used for the test, so: n=2r-1r=n-k

Therefore, bit-wise interleaving is most beneficial for Hamming code verification. This type of disc is suitable for reading and writing big data. However, the redundant information overhead is still too large, preventing the widespread use of such disks.

4. RAID3 (3 level disk array)

RAID3 is a single-disk fault-tolerant parallel transmission array disk. It is characterized by reducing the inspection disk to one (the RAID2 check disk is multiple, the DAID1 check disk is 1 to 1), and the data is stored in each disk in bits or bytes (distributed in the same sector number in the group) On each disk unit). Its advantage is that the bandwidth of the entire array can be fully utilized, so that the bulk data transmission time is reduced; the disadvantage is that each read and write can affect the entire group, and only one I/O can be completed at a time.

5.RAID4 (4th level disk array)

RAID 4 is an array that can read and write to each disk in a group independently. There is only one check disk.

The difference between RAID4 and RAID3 is that RAID3 is interleaved by bit or byte, while RAID4 is accessed by block (sector). It can operate on a disk separately. It does not need to be like RAID3, even if it is A small I / O operation also involves the entire group, only need to involve two disk units in the group (one data disk, one inspection disk). Thereby increasing the I/O rate of a small amount of data.

6. RAID5 (5-level disk array)

RAID 5 is an array of rotating parity independent access. It differs from RAID 1, 2, 3, and 4 disk arrays in that it has no fixed parity disk, but distributes its redundant parity information evenly on all disks to which the array belongs according to certain rules. . Therefore, there are both data information and verification information on the same disk unit. This change solves the problem of contention parity disks, so multiple writes are allowed concurrently within the same group in DAID5. Therefore, RAID5 is suitable for large data operations and is also suitable for various transaction processing. It is a fast, large capacity and fault tolerant distribution of disk arrays.

7. RAID6 (6-level disk array)

RAID6 is a two-dimensional parity independent access disk array. Its redundant detection and error correction information is evenly distributed on all disks, and the data is still stored in each disk in a cross-over manner with variable-sized 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. Cache is a kind of cache memory, that is, data is written into the cache before being written to the disk array. Generally, the size of the cache block is the same as the size of the data block in the disk array, that is, one block of the cache corresponds to one block of the disk. 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 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 many blocks of data being written 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 disk read operations, which makes full use of the disk bandwidth.

This combination of cache and disk array technology makes up for the shortcomings of disk arrays (such as poor block response request response), so that the entire system is provided with efficient, fast, large capacity, high reliability and flexible and convenient storage system. Users, thus meeting the needs of current technological developments, especially the needs of multimedia systems.