Thanks Joe, I've been reading about Raid for 15 years and still don't get it and that did not change with you post!
The nice thing about RAID is it protects your data. Here's how it works:
Suppose you write a letter to your mom. It starts with "Dear mum, life is fine." You save it as a file on your hard drive. That means somewhere on your hard drive is stored a series of characters: D,e,a,r, ,m,u,m...
Now the hard rive fails, for whatever reason. Maybe a bit of falling debris from the space station lands on it, whatever. That hard drive is destroyed; everything is gone. So far, so simple.
Now, a RAID array is a whole bunch of hard drives connected to a special controller so the computer thinks they are just one hard drive.
Let's say you have 4 hard drives in your RAID array. You save a letter to your mum. There are now data stored on your hard drives, but in a very special way:
Hard drive #1 has "D". Hard drive #2 has "e". Hard drive #3 has "a". Hard drive #1 has "r." Hard drive #2 has " ". Hard drive #3 has "M", hard drive #2 has "u", hard drive #3 has "m", hard drive #1 has "," hard drive #2 has " ", hard drive #3 has "l", hard drive #2 has "i", hard drive #3 has "f", hard drive #1 has "e"... Basically, the file is broken up into pieces and little tiny chunks of it are stored on three of the hard drives.
What is stored on the fourth hard drive? Ah, that's the magic.
The fourth hard drive stores the result of mathematical operations on the first three hard drives. So you end up with your letter splintered up in small pieces across three hard drives, and special codes on the fourth hard drive that describe mathematically what is on the first three.
Why do this?
Say that one of the four hard drives--we'll call it #2--dies.
Now the RAID array automatically reverses the mathematical equations that were used to create the data on hard drive #4. So you go to read the letter to your mum, and here's what happens:
It reads the file from the hard drives. But hard drive #2 has been obliterated, so the file looks like this:
D?ar?Mu?, ?if? i? f?ne
Pretty scrambled, right? Every third letter is missing! The file was broken up across three hard drives and now one of them is gone.
So the RAID controller, which is actually a small computer, looks at the information on hard drive #4. It says "I have the letters D and a, but the letter in between is gone because that hard drive is gone. Based on the letters D and a and the mathematical result that was stored on you, what is the missing letter?" And it calculates the answer: It must have been the letter e.
Then it looks at the next three letters. Based on the fact that the next letters are the letter r, then a missing letter, and then the letter M, what must the missing character be? And it does the math and calculates that the missing letter must be a space.
It does this over and over again and it comes up with D,e,a,r, ,M,u,m, ,l,i,fe, ,i,s, ,f,i,n,e. It uses the special codes written on hard drive #4 to rebuild what was lost when hard drive #2 was destroyed.
