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When someone mentions RAID in a conversation about backups, invariably someone declares that "RAID is not a backup."

Sure, for striping, that's true. But what's the difference between redundancy and a backup?

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13 Answers 13

up vote 81 down vote accepted

RAID guards against one kind of hardware failure. There's lots of failure modes that it doesn't guard against.

  • File corruption
  • Human error (deleting files by mistake)
  • Catastrophic damage (someone dumps water onto the server)
  • Virus'
  • Software bugs that wipe out data ..
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Will a backup refuse to copy a corrupt file? –  jldugger May 2 '09 at 0:53
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Depends on what "corrupt" means but normally backup applications have a setting for this... however, the second point of backup is to keep different versions of the file through time - not just a single version - thus circumventing the problem with a newly corrupted file overwriting a fresh version... –  Oskar Duveborn May 2 '09 at 1:00
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> Will a backup refuse to copy a corrupt file Yes, if you cannot read the blocks of a corrupt file, you won't be able to make a copy of it (backup) –  Dave Cheney May 3 '09 at 3:22
    
But what about silent data corruption; if a data block goes bad, most filesystems won't notice, will they? –  jldugger May 3 '09 at 18:07
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Reasonable backup strategies include keeping a history, so that you can go back to before the corruption. The most common handling of the possibility of corruption is to pretend it can't happen. But if you want to protect against it, you can attempt to detect it as soon as possible, and in varying chunk sizes (device block level, database page level, file level). If you detect data corruption fast enough, it isn't "silent" data corruption anymore and you have a chance of recovery. –  carlito Jun 1 '09 at 20:04

Q: Why is RAID not a backup?

A: Because the whole purpose of a RAID is to make sure that nothing in the world can interrupt that accidental rm -rf / (or DELTREE /X C:\), not even yanking the power chord in panic.

Q: But whats the difference between redundancy and a backup?

A: If you accidentally overwrite your PhD thesis with garbage, redundancy ensures that you have multiple copies of garbage, in case one gets bad. A backup ensures that you can restore your PhD thesis.

(And an archive ensures that you can retrieve multiple older versions of your thesis, and a version control system also tells you why you made a new version in the first place.)

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Redundancy protects you against your hardware failing. It does not protect against user error, nor against malicious activity (e.g., crackers getting into your system).

See: Why Mirroring is Not a Backup Solution for a hard-earned lesson.

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Nor software bugs, which are more common than malicious activity. –  jhs May 3 '09 at 14:39
    
It's an interesting bit of irony that the article linked from that Slashdot page has now disappeared off the web. Not even the Internet Archive provides a meaningful copy; even though they did crawl the page shortly after the Slashdot article date, their copy simply says the page was not found. –  Michael Kjörling Dec 24 '13 at 15:11

The number one reason you want a backup is not because the physical media died (this is rare), but because of some error that caused the data to be lost or corrupted.

RAID doesn't protect you against a file being deleted.

RAID doesn't protect you against a file being overwritten.

RAID doesn't protect you from your system being compromised and all of your data being overwritten, deleted, or corrupted.

RAID doesn't protect you from your ops team accidentally paving a machine with important data on it.

RAID doesn't protect you from a foolish DBA running a drop command on the production server (mistaking it for a test environment).

RAID doesn't protect you if the building burns down.

P.S. http://ma.gnolia.com/. This is what can happen if you don't have good backups. Your site is snuffed out of existence (note: this tends to be bad for business).

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Redundancy is great if one of your disks fails. It's no so great if your computer gets a virus, or you mistakenly delete a file, or you need to restore the disk to a previous version for some other reason. That's when you need a backup.

RAID helps you recover from failures, but backups let you go back in time.

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RAID can be a great way to mitigate risks due to hardware failures, but RAID won't help you when your users delete (accidentally or otherwise) their data. To recover data you need some archival facilities, either through local snapshots or online/offline backups.

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It should also be mentioned that a hardware fault in the raid controller can easily corrupt the data on all attached disks. So while you reduce the danger from disk failures you add the danger of raid controller failures.

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  • Multiple rotating copies
  • Geographic redundancy

Asked in a comment to the accepted question:

Will a backup refuse to copy a corrupt file?

Even if a backup copies corrupt or bad data, the point of a backup is that you can and should have multiple copies. For instance, last hour, yesterday, last week, etc. You can get a similar effect from using rotating snapshots on your storage device.

But the other reason for backups is geographic redundancy. You should certainly keep copies of critical data in two different geographic locations. How separate those locations are depends on how critical the data is; keeping copies in two different buildings in the same city protects against fire or theft. Keeping copies in two different countries protects against bigger problems.

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In a RAID5 array, consisting of disks over 400Gb, if you lose a disk there's something like a 75% chance of having an unrecoverable read error while the array is being rebuilt. Think about that for a second and it becomes pretty obvious why someone will always remind you that "RAID is not a backup".

RAID gives you higher reliability and performance, but it's not infallible.

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Real problem, bad math. –  Paweł Brodacki Sep 11 '11 at 16:12

Fire, theft, RAID controller fault, human error, the list goes on

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Also consider with raid that you have multiple hard drives probably build at the same time and then exposed to the same conditions for years .... what are the chances that they will all fail about the same time .... pretty high

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MTBF != expected lifespan of gear –  Tetsujin no Oni May 7 '09 at 20:38
    
This isn't really an issue with RAID, though. Well, the "same use patterns" might be exacerbated by RAID, but multiple drives exposed to the same conditions isn't a function of RAID. –  Michael Kjörling Dec 24 '13 at 15:17

What's the difference between redundancy and backup? Ok, configure a RAID 5 disk set. Store some business-critical stuff on it. Pull a disk out. Everything still works! That's redundancy. Now delete all the data (don't cheat with the recycle bin). Now restore it from the most recent backup. You don't have one? Oops. Well at least you can tell your boss your disks are using RAID 5 redundancy (as you get marched out of the building...)

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