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When reading about RAIDs, you are almost always going to stumble about the fact that RAID is not a backup.

So, what are the reasons to use RAID then?

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closed as not constructive by Ƭᴇcʜιᴇ007, terdon, soandos, Breakthrough, Dave M Jun 10 '13 at 12:21

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"R" is for Redundant. –  Ƭᴇcʜιᴇ007 Jun 8 '13 at 19:12
    
I +1, I think the question shows a lack of research and effort... but the format was clear and concise for a good QA format so I +1. –  AthomSfere Jun 8 '13 at 19:25
    
I do know about some of the reasons (e.g. what has been said in the 2 answers posted until now), but I wanted to keep the question open to utilise the Q/A format better. The question is still open to additional contributions of course. –  Zulakis Jun 8 '13 at 20:24
    
@techie007 likewise, I find it humorous that RAID 0 has no (or zero :) redundancy. –  Breakthrough Jun 9 '13 at 6:10

4 Answers 4

Hard drives are one of the most common parts of a computer that fail. This is due to the fact that they have moving parts. If a computer has one hard drive and it fails, the data is lost and the computer is non-functional.

RAID makes use of multiple hard drives to make a redundant system. If one or more drives, depending on setup, are lost the computer will still be able to function with no downtime or loss of data.

Why do we say "RAID is not a backup?" This is because multiple disks in a RAID array act as one single disk. If you were to format the logical drive that a RAID creates, you have just erased all your data. If you were to write bad data to the drive, you can not undo this and would have to restore from backup.

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So that, if when a disk drive fails, your computer isn't dead in the water with complete data loss/corruption, but continues to run until you replace it.

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Are there any reasons to use RAID besides having to recover from a backup without it? –  Zulakis Jun 8 '13 at 19:15
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@zulakis It's all about up-time, and access speeds. As you have seen: RAID IS NOT FOR BACKUPS. It has nothing to do with data backup, aside from keeping you from needing to restore data in the event of hardware failure. Go read en.wikipedia.org/wiki/RAID it gives the reason(s) to use RAID in the first 3 paragraphs. :) –  Ƭᴇcʜιᴇ007 Jun 8 '13 at 19:18
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And some RAIDs perform no redundancy, only speed... and some give the OS the illusion of a single larger drive... –  AthomSfere Jun 8 '13 at 19:24
    
In a setup where partial downtime to do a backup recovery is no problem and access speed is fine, there is no reason to use RAID then? –  Zulakis Jun 8 '13 at 19:41
    
attention @techie007: if you want to answer the question, then answer the question. I don't need you changing my answer. –  hymie Jun 10 '13 at 18:17

There are several types of RAID but they all serve one or both of two tasks

  1. Increase disk throughput (Speed)
  2. Decrease downtime in the event of a hardware failure.

Some raid levels, like RAID 0, do not help you decrease downtime, in fact the increase the chances of downtime because you now have 2 or more disks that could fail and if one does fail you need to restore the whole image from a backup (you could re-use the non failed disks though once you replaced the failed drive).

Other raid levels are mostly for redundancy, like RAID 1, you will have increased read performance in line with the number of disks you have, but write performance will be the same as if you only had your original disk. If one of the drives suffers a hardware failure, the system will remain up and running and the only bad side effect you would see is increased read times due to the disk that is offline no longer participating.

All other types of RAID are just trying to balance the two tasks with different weights on task #1 or task #2.

Now to clarify the "not for backup" part. If I delete a file on my RAID system, all RAID will do is make the delete happen very fast, nothing RAID provides helps me recover that file after it has been deleted, the only thing I can do is restore from a backup. A similar story with a virus infection, the virus will "infect" all of the drives, there is no way to undo the changes it did.

The basic summery is: backups let you "roll back" to a earlier point in history, RAID (if used for task #2) lets you "roll back" less often due to hardware failure. Raid offers no protection from User or Software error, but it's not supposed to that is what backups are for.

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When I architected systems, there were a few different configurations that we used. Everyone above is correct that raid improves the fault-tolerance of your system. So if you have a hard drive failure, your data is preserved because it is mirrored across hard drives. This is important if your application is mission critical and the failure of the hard drive would shutdown the server.

Second, if you need to run a system that has a lot of read-write throughput, like a database server or a machine for rendering 3D images/video, then raid will allow you to write to multiple drives at the same time. So that way a single drive head does not have to keep moving all across the disk--any of the drive heads can write the data to the disk, based upon the raid configuration.

The key question is what other backup stuff do you need in addition to raid. So raid systems don't protect you from viruses. So if a virus infects your system and you actually need to rebuild your computer, then the raid drives will not save you--they will all be infected. So you still need a backup solution, such as tape backup, a storage management unit like EMC, or some sort of online storage that backup up your hardware continuously.

Hope this helps.

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