Having a number of disks in RAID may seem like a backup, especially if you’re using a mirror mode. But this is wrong!

RAID protects you against one and only one thing: a disk failure.

It does not protect you against any of the following things:

• Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6), including possible UREs - on that later subject, RAID-5 should be considered harmful these days for any disks larger than 1TB.
• Failure of the RAID controller itself (especially when using hardware RAID), the computer running the RAID, or the environment containing the servers (a flood, fire, theft, etc.).
• Data corruption on-disk from filesystem bugs, cosmic rays, or minor hardware or firmware failures, which can and do happen all the time - you usually just don’t notice and software works around it.
• Malicious or accidental deletion or modification of files by yourself or another party, including viruses, bad application writes, or administrative mistakes (e.g. rm-ing the wrong file or mkfs on an existing filesystem), which any seasoned sysadmin has done at least once (and hopefully not to production data)!

The adage is simple: “RAID replicates everything, instantly, even the stuff you don’t want it to.”

There exists a number of file and storage systems with some advanced, RAID-like features. These include ZFS, btrfs, and Ceph. On the surface, these might give you the illuson of protection, but don’t be deceived. You can still trash your whole system (or cluster, for Ceph). You can still rm files or run other destructive commands accidentally. A fire can still destroy your whole rack. A malicious user could overwrite your database. Even the smartest most advanced storage engine is still susceptable to at least one, and almost always several, fatal failure modes.

Like RAID, ADVANCED FILESYSTEMS STILL AREN’T BACKUPS! Just make another copy of the data, okay!?

• Always back up in some way. While a copy of the data on the same array won’t protect you against all, or even very many, failure modes, it will protect you against some, and those are usually the most common.
• A backup on the same server is susceptable to the same failures as the original data set (hardware failure, natural disasters, and the like).
• A good rule of thumb is three copies: the original (RAID or otherwise); one onsite copy on a different, preferrably offline, medium; and one offsite copy. Store the offsite copy in the cloud, a data vault, or at a friend’s house, just keep it somewhere else.
• Make backups regularly, at least once a week, preferrably more, and automate it! Forgetting to back something up and then needing just that backup is never fun, and the more frequently you back up, especially incrementally, the better your recovery resolution.
• Test backups regularly, at least once a month; a backup is worthless if you can’t restore from it. Just because you have a backup doesn’t mean you’re protected; always test them and fix any problems. If you never test your backup, you will almost certainly find it doesn’t work, right when you need it.

There are dozens of backup utilities out there; I’m not going to prosthelytize for any one of them, but I personally use BackupPC and good ol' fashoned rsync for my server and workstation backups.

Only you can determine what you need to back up, but if you can’t replace some data, you should definitely back it up - Murphy’s Law applies here as much as anywhere.

Now that you’re in the know, get to making and checking a backup of your data, before you lose it!

More information can be found on the following pages:

• http://www.smallnetbuilder.com/nas/nas-features/31745-data-recovery-tales-raid-is-not-backup
• http://blog.open-e.com/why-raid-is-not-a-backup/
• http://serverfault.com/questions/2888/why-is-raid-not-a-backup