UNIX System Management: RAID and disk arrays

This is the technology that protects data integrity. RAID means redundant cheap http://www.aliyun.com/zixun/aggregation/20901.html "> disk array. It is a rather new concept, presented by the University of California, Berkeley, in 1987.

The basic idea behind raid is that by using multiple small disks (a relatively small number of large disks) and possibly some checksum data, you should be able to reconstruct the data instead of losing a few G data in case one of the disks fails.

There are several different levels of raid, numbered from 0 to 5. Their respective functions are given in the table below. Some high-end UNIX machines have disk arrays that are implemented with hardware-level RAID technology, while on some low-grade machines, such as Linux, software can be used to implement raid. But the performance is worse.

Level 0 (data strip) This does not provide any real protection for data loss, but it greatly improves performance (performance is very good but not safe). Requires a controller for a synchronous disk

Level 1 (disk mirroring) The second set of disks is used to provide a complete data backup. Security is very good disadvantage is more expensive because it takes twice times the disk

Level 2 has a checksum disk based on level 0, which is responsible for storing error-correcting information. But the performance is not very good.

Level 3 has a separate disk on level 0, processing byte-level checksum error-correcting information to be used to reconstruct the data.

If you verify the disk corruption, you lose the integrity of the data. Synchronized disks can break the bottleneck limit for a single checksum disk performance

Class 4 Level 3 is based on a block checksum, stored on a separate disk. Read performance is worse than level 3. Because it is not used

Sync disk, which can be implemented with software

Level 5 is based on level four, but its checksum information is distributed across all disks, and performance is good because a single checksum disk is no longer a bottleneck. But the writing performance is still not as good as Level 3