Overview of RAID Levels
The Berkeley engineers defined 5 different RAID levels. These levels are not ratings, but rather classifications of functionality. Different RAID levels offer dramatic differences in performance, data availability and data integrity depending on the specific I/O environment. There is no single RAID level that is perfect for all users.
RAID level 0 refers to striping data across multiple disks without any redundant information. RAID level 0 was not defined by the Berkeley engineers but has become a commonly used term.
Striping can be used to enhance performance in either a request rate intensive or transfer rate intensive environment. Unfortunately. striping reduces the level of data availability since a disk failure will cause the entire array to be inaccessible.
RAID level 1 refers to maintaining duplicate sets of all data on separate disk drives. Of the RAID levels, level 1 provides the highest data availability since two complete copies of all information are maintained. In addition. read performance may be enhanced if the array controller allows simultaneous reads from both members of a mirrored pair. During writes, there will be a minor performance penalty when compared to writing to a single disk. Higher availability will be achieved if both disks in a mirror pair are on separate I/O busses, known as duplexing.
Peer-to-Peer RAID 1: Mirroring over Networks
Peer-to-peer RAID Level 1 refers to maintaining duplicate sets of all data on separate disk drives, on separate hosts or servers. Of the RAID levels, this provides the highest data availability since two complete copies of all information are maintained. During writes, there will be a minor performance penalty when compared to writing to a single disk.
In RAID level 3, data is striped across a set of disks. In addition, parity is generated and stored on a dedicated disk. With RAID 3, data chunks are much smaller than the average I/O size and the disk spindles are synchronized to enhance throughput in transfer rate intensive environments. RAID 3 is well suited for CAD/CAM or imaging type applications. Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.
In RAID level 5, both parity and data are striped across a set of disks. Data chunks are much larger than the average I/O size. Disks are able to satisfy requests independently which provides high read performance in a request rate intensive environment. Since parity information is used, a RAID 5 stripe can withstand a single disk failure without losing data or access to data.
Unfortunately, the write performance of RAID 5 is poor. Each write requires four independent disk accesses to be completed. First old data and parity are read off of separate disks. Next the new parity is calculated. Finally, the new data and parity are wntten to separate disks. Many array vendors use write caching to compensate for the poor write performance of RAID 5.