Something For Everyone: Possible Applications
Though RAID Level 0 is the fastest option of all, it is also the most precarious by far. For example, using four hard drives will push the data transfer rate far beyond 100 MB/s, but fault tolerance will be virtually non-existent. A hard drive is a mechanical component that will age and wear out after a while. Mechanically-induced defects are therefore really only a question of time. But even an electronic error or a minor production error may result in a catastrophe.
For this reason, RAID 0 is not recommended for long-term storage data, but primarily for setting up fast drives with temporary data, such as file or database servers. And if the system has to be mirrored on short notice, RAID 1 is your best choice. If you have a hot swap cabinet, you can remove the hard drive while the system is running in order to mirror it to a different drive of the same size on a different computer. Then the drive is reinstalled in the computer, while the copy can be saved once again with a RAID 1 when connecting it to a RAID controller.
RAID Level 1 does nothing other than mirror a hard drive's data (in special cases, also those of any array) to another hard drive in real-time. People are inclined to believe the misleading proposition that RAID 1 does not offer any improvement in performance. Though write operations really aren't any faster than with only one hard drive, when reading data, it is possible, in theory, to have a data transfer rate equivalent to that of an analog RAID 0. This is only logical, since data can be read simultaneously from all the drives in the array. In practice, however, there are differences, as the data to be read are not available in cleanly split stripes as they are with RAID 0; instead, the controller has to perform this division itself based on specific patterns.
Using RAID 1 makes sense if your main focus is on maximum data security and minimum recovery efforts (e.g., simple servers). Most RAID controllers are able to perform the recovery procedure independently after a hard drive has been exchanged. You can do this on the fly only if the hard drives are housed in hot swap cabinets.
RAID Level 3 is losing more and more of its popularity because RAID 5 offers the same advantages with fewer disadvantages. With RAID 3, parity data is written to one or maybe even several hard drives. The big advantage is in the distribution of the actual data stock to several drives, in the form of stripe sets, actually allowing a significantly higher data rate - and at the same time protecting against a hard drive crash. Its disadvantage, however, is the fact that the parity data is written to only one drive. This cuts down on write performance considerably.
RAID 3 is usually deployed in servers with mostly static data or servers that require better performance than RAID 0 can provide, without foregoing data security. This is a simple way to keep the low write performance from carrying too much weight.
RAID Level 5 dominates in today's high-end server segment. If you're using four to seven drives, such an array is a real performer and, if the drives are large, allows accordingly large partitions. Unlike RAID 3, the parity data are integrated in the stripes on all drives and are distributed in a way that will have a positive impact on performance. Consequently, RAID 5 offers a high level of performance for all kinds of applications.
