Server learning-raid and SCSI

First, we will introduce scsiscsi, which is called "Small Computer System Interface" in English and the Chinese name "Small Computer System Interface ". It is a peripheral interface, which is mainly used by hard disks on servers. In addition, this interface is also used for CD/DVD-Rom, CD-r/RW, scanners, and tape drives. In fact, SCSI is not a new interface type. As early as 1986, the SCSI standard was developed and has been around for more than 20 years. The SCSI interface is expensive and has always been known for its high transmission rate and high reliability. It is widely used in servers and high-end PCs. We often say that server hard disks are hard disks with SCSI interfaces. Compared with the commonly used IDE (ATA) interface in PC (currently the fastest is 133 Mb/s), its transmission rate has obvious advantages, therefore, the server usually uses a SCSI interface hard disk, which is a very common ide interface hard disk. However, the transmission rate of the new SATA (Serial IDE) interface is close to that of the SCSI interface and is being applied on the server.

 

Next we will introduce raid

Raid is short for "redundant arrayof independent disk", which means "independent redundant disk array" in Chinese ". A simple explanation is to combine n hard disks with RAID Controller (hardware, software) into a single virtual large-capacity hard disk. Disk Arrays use different technologies for different applications, called raid level, and each level represents different technologies, currently, the definition of RAID level is widely recognized by the industry in four categories: RAID 0 and raid
1. RAID 0 + 1 and RAID 5. This level does not represent the level of technology. To select a raid level product, it depends solely on the user's operating environment (operatingenvironment) and application. It is not necessarily related to the level. RAID levels can be combined or extended to form RAID levels such as raid 10, 50, and 60.

Here we will introduce the different levels of RAID level:

RAID 0: error-free zone. RAID 0 is the fastest speed. However, RAID 0 does not have redundancy. If a disk (physical) is damaged, all data cannot be used.

Raid 1 is a complete image of two hard disks, featuring high security, simple technology, convenient management, and good read/write performance. Because it is one-to-one, it cannot be extended by a single hard disk. To expand it, both sides of the image must expand the same capacity at the same time. This redundancy method uses only half of the disk capacity for security purposes, resulting in a large waste of data space. RAID 0 + 1 combines the features of RAID 0 and RAID 1. An independent disk is configured as raid 10, and two sets of complete RAID 1 0 mirror each other. It provides excellent read/write performance and high security, but it costs a lot to build an array and has low data space utilization. RAID 5 is the most widely used RAID technology. Each Independent hard disk is segmented by strip, and the same strip area is used for parity (exclusive or operation). The parity data is evenly distributed on each hard disk. The RAID 5 array built with N hard disks can have n-1 hard disks with high storage space utilization. Data loss on any hard disk can be calculated by verifying the data. The biggest difference between it and RAID 3 is that it verifies whether data is evenly distributed to each hard disk. RAID 5 has the advantages of data security, fast read/write speed, and high space utilization. It is widely used, but the disadvantage is that if one hard disk fails, the performance of the entire system will be greatly reduced.

Raid 1, RAID 0 + 1, RAID 5 array combined with hot swapping (also known as Hot replaceable) technology, can achieve online data recovery, that is, when any hard disk in the raid array is damaged, if you do not need to shut down or stop the application service, you can replace the faulty hard disk, repair the system, and restore data, which is of great significance to the high-availability system.

The reason why servers are selected as raid disk arrays is that they have the following advantages: Support for automatic detection of faulty hard disks; support for reconstruction of Hard Disk Bad track data; Support for hard disk backup without downtime hotspare; supports replacement of Hot Swap with hard disks that do not require downtime; supports expansion of hard disk capacity.