I need help understanding RAID. I know and understand that RAID 1 is disk mirroring. Copies one hard drive onto the other. However I cannot figure out what exactley RAID 5 is. I know this is very commonly used by web hosts but how exactley does it work? How many drives do you have to have?

just a small sampling... and "n" is the number of drives

RAID0 - (striping)any number of drives, all lined up together to make one huge drive. no redundancy.. if any drive dies you're screwed. speed increase.. data is split across each of the drives, read time is improved, write time should be improved. storage space = n * drivesize

RAID1 - (mirroring)at least 2 drives. all data is written across every drive. any number up to n-1 drives can fail without loosing data. read time is generally improved because the data can be read synchronously from any of the drives. storage space = n/n * drivesize (or, just drivesize)

RAID5 - (striping with parity)at least 3 drives. data is split into n-1 pieces and each piece is written to one drive of the array. a checksum is computed of each piece of data, and is written to the last drive as parity. only 1 drive can fail at a time without loosing data. space = n-1

RAID 10, 1+0, 0+1 - at least 4 drives. combines RAID0 and RAID1. this is multiple raid1 arrays mirrored via raid0. speed increase.. data is seperated(striped) to each mirror instead of to each drive. read time improved, can read from any drive in each mirror, and is read from both striped sets. a certain number specific drives can fail without loosing data. this number will vary based on configuration. 4 drives, 2 stripe sets, mirrored: 1 drive from either stripe set can fail, but not both. 1 drive can fail from both stripe sets. this is the most expensive configuration, space = n/2 * drivesize

edit: i had the words "(striping)" and "(mirroring)" in the wrong place... they were last minute additions... sorry... thx muppie

So you think we should use RAID 1? Is it the most usful for web hosting?

Correction,

RAID0 is striping without parity, and RAID1 is mirroring.

What you want to choose depends on what you want to achieve. RAID1 / mirroring tends to be slower, but more reliable. RAID0 offers no reliability, only speed.

For more complete information, check out:
http://www.raidweb.com/whatis.html

btw for some reason I can't open that url :( you guys have any luck? If that url is not working just do a google search for raid :)