RAID Basics

People refer to a RAID0 implementation as striping. A simple example of how this works is as follows: Imagine you are filling a bath tub with water to a certain level from a small tap that's on as fast as it can go. The bath tub will be full to that level in x amount of time. If we want to fill the tub faster we can add a second tap and it will theoretically fill twice as fast. Now assume you have a 500MB file (water) you have to write to your 40GB hard disk drive. The file will take x time to write to disk because the disk can go so fast but not any faster (tap size). Now instead of writing the 500MB file to a single disk imagine if we wrote half of the file (250MB) to two (add a second tap) 20BG Hard disks. Theoretically it should also be tow times as quick! This is how RAID increases performance. Chunks (blocks) of KB are written to each disk. So now the minimum time taken to write 500MB to one disk is halved because only 250MB has to be written to one disk. If we add yet another disk (another tap) only ~166MB needs to be written to each disk and so we add more disks. It is ideal to use the same type and size of hard disk when striping. If different sizes are used the total RAID0 capacity is no of drives in array times smallest disk capacity. We should avoid wasted capacity. RAID0 adds no redundancy. RAID0 is mainly used is performance critical applications where redundancy is not required or is implemented via another means. It is becoming popular is small office and home PC's.
             RAID level 1 is the second implementation of this technology. It is also called mirroring or shadowing. This is where data redundancy comes into play in the RAID system. RAID1 is a simple implementation as it is basically just a backup of the first disk. If we have a 2 hard disk RAID1 mirror then disk 2 is an exact image or copy of disk one. If disk one or two fails the system can keep running and the broken disk can be replaced for continued redundancy.