[Hivetyrant]

Here is a decent description of RAID 5 and its disadvantages/advantages

RAID 5 RAID 5 attempts to combine the benefits of RAID 0 and RAID 1, while minimizing their respective disadvantages. Like RAID 0, a RAID 5 array consists of multiple disk drives, each divided into chunks. This allows a RAID 5 array to be larger than any single drive. And like a RAID 1 array, a RAID 5 array uses some disk space in a redundant fashion, improving reliability. However, the way RAID 5 works is unlike either RAID 0 or 1. A RAID 5 array must consist of at least three identically-sized disk drives (although more drives may be used). Each drive is divided into chunks and data is written to the chunks in order. However, not every chunk is dedicated to data storage as it is in RAID 0. Instead, in an array with n disk drives in it, every nth chunk is dedicated to parity. Chunks containing parity make it possible to recover data should one of the drives in the array fail. The parity in chunk x is calculated by mathematically combining the data from each chunk x stored on all the other drives in the array. If the data in a chunk is updated, the corresponding parity chunk must be recalculated and updated as well. This also means that every time data is written to the array, two drives are written to: the drive holding the data, and the drive containing the parity chunk. One key point to keep in mind is that the parity chunks are not concentrated on any one drive in the array. Instead, they are spread evenly through all the drives. Even though dedicating a specific drive to contain nothing but parity is possible (and, in fact, this configuration is known as RAID level 4), the constant updating of parity as data is written to the array would mean that the parity drive could become a performance bottleneck. By spreading the parity information throughout the array, this impact is reduced. Advantages to RAID 5 Compared to a single drive, a RAID 5 array has the following advantages: *Improved redundancy — If one drive in the array fails, the parity information can be used to reconstruct the missing data chunks, all while keeping the data available for use *Improved read performance — Due to the RAID 0-like way data is divided between drives in the array, read I/O activity is spread evenly between all the drives *Reasonably good cost efficiency — For a RAID 5 array of n drives, only 1/nth of the total available storage is dedicated to redundancy Disadvantages to RAID 5 Compared to a single drive, a RAID 5 array has the following disadvantage: *Reduced write performance — Because each write to the array results in two writes to the physical drives (one write for the data and one for the parity), write performance is worse than a single drive[2]

[ 09-12-2005, 11:36 PM: Message edited by: Hivetyrant ]