Why is RAID not a backup?
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?
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)
- Viruses and other malware
- Software bugs that wipe out data
- Hardware problems that wipe out data or cause hardware damage (controller malfunctions, firmware bugs, voltage spikes, ...)
and more.
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.)
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.
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).