RAID is the abbreviation of redundant arrays of inexpensive disks in English and redundant arrays of inexpensive disks in Chinese. RAID is a redundant array composed of multiple hard disks. Although RAID contains multiple hard disks, it appears as an independent large storage device under the operating system. Using RAID technology in storage system has three main benefits:
1. provides disk spanning function by organizing multiple disks into a logical volume;
2. Write/read data into/from multiple disks in parallel by dividing data into multiple data blocks to improve the speed of accessing disks;
3. Provide fault tolerance through mirroring or verification operations.
The main purpose of RAID's initial development is to save costs. At that time, the total price of several small-capacity hard disks was lower than that of large-capacity hard disks. At present, the role of RAID in cost saving is not obvious, but RAID can give full play to the advantages of multiple hard disks, and its implementation far exceeds the speed and throughput of any single hard disk. In addition to improving performance, RAID can also provide good fault tolerance, and it can continue to work in the case of any hard disk failure, which is not affected by hard disk damage.
RAID technology is divided into several different levels, which can provide different speed, security and cost performance. Choosing the appropriate RAID level according to the actual situation can meet the user's requirements for the availability, performance and capacity of the storage system. The commonly used RAID levels are: NRAID, JBOD, RAID0, RAID 1, RAID0+ 1, RAID3, RAID5, etc. At present, RAID5 and RAID(0+ 1) are commonly used.
The working principle of RAID 0
As shown in the figure, the I/O data request sent by the system to a logical hard disk (RADI 0 disk group) composed of three disks is converted into three operations, and each operation corresponds to a physical hard disk. From the figure, we can clearly see that by establishing RAID 0, the original sequential data requests are distributed to all three hard disks for simultaneous execution. Theoretically, three hard disks can run in parallel, which can simultaneously improve the disk reading and writing speed by three times. However, due to the influence of bus bandwidth and other factors, the actual speed will definitely be lower than the theoretical value. But there is no doubt that the speed-up effect of parallel transmission of a large number of data is remarkable compared with serial transmission.
Advantages and disadvantages of RAID 0
The disadvantage of RAID 0 is that it does not provide data redundancy, so once the user data is damaged, the damaged data cannot be recovered.
The characteristics of RAID 0 make it especially suitable for fields that require high performance but don't care much about data security, such as graphics workstations. For individual users, RAID 0 is also an excellent choice to improve the storage performance of hard disks.
Discussion on disk array technology and its development trend
With the development of computer technology, the processing speed of CPU jumps geometrically, and the access speed of memory is greatly improved, but the access speed of disk is very slow. The whole I/O throughput can't keep up with the system, which forms the bottleneck of the whole computer system and reduces the overall performance of the computer. In order to improve the access speed of disks, RAID technology (cheap redundant disk array) is often used for disks of large servers.
RAID technology combines multiple physical disks into an array as a logical disk group, and stores data on different physical disks of this logical disk group in a segmented manner. When accessing data, the related disks in the array work in parallel, which greatly reduces the time of data access and makes the space utilization better.
A great reason for the development of RAID technology is that the data of many large institutions are very valuable, such as bank deposit and withdrawal information, telecom charge records, taxpayer files in State Taxation Administration of The People's Republic of China and so on. These institutions should consider the security of data when implementing enterprise informatization. At present, many servers will work continuously. Due to long-term service, some disks will fail. Without protective measures, a lot of important data will be lost. In this case, the fault-tolerant function of the disk array can protect the security of these important data.
In order to strengthen the fault-tolerant function, so that the system can quickly rebuild data when the disk fails, thus maintaining the performance of the system, the general disk array system adopts the function of hot backup. The so-called hot backup is to designate one of the disks as a backup disk when establishing a disk array system. This disk is not working properly. When a disk in the array fails, the disk array immediately replaces the failed disk with a backup disk and automatically reconstructs the data of the failed disk on the backup disk. Because of the fast response speed, the access of memory to disk is reduced, so the data reconstruction can be completed quickly, which has little impact on the system performance. Hot backup is an important function for a large data processing center or control center that requires no downtime, because it can avoid all kinds of inconvenience caused by disk failure at night or unattended.
4 kinds of disk arrays
RAID is an industrial standard, and different manufacturers have different definitions of RAID levels. At present, there are four widely recognized definitions of RAID levels in the industry: RAID 0, RAID 1, RAID 5 and RAID 7. RAID 0: Striped disk array without fault-tolerant design.
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