Hard disks fail. If you prepared properly, you have a backup, or you swap out disks when they first start reporting problems. If you prepare really well you have offsite backup (which is getting easier and easier to do over the internet.)

One way to protect yourself from disk failures is RAID, especially RAID-5. With RAID, several disks act together as one. The simplest protecting RAID, RAID-1, just has 2 disks which work in parallel, known as mirroring. Everything you write is copied to both. If one fails, you still have the other, with all your data. It’s good, but twice as expensive.

RAID-5 is cleverer. It uses 3 or more disks, and uses error correction techniques so that you can store, for example, 2 disks worth of data on 3 disks. So it’s only 50% more expensive. RAID-5 can be done with many more disks — for example with 5 disks you get 4 disks worth of data, and it’s only 25% more expensive. However, having 5 disks is beyond most systems and has its own secret risk — if 2 of the 5 disks fail at once — and this does happen — you lose all 4 disks worth of data, not just 2 disks worth. (RAID-6 for really large arrays of disks, survives 2 failures but not 3.)

Now most people who put in RAID do it for more than data protection. After all, good sysadmins are doing regular backups. They do it because with RAID, the computer doesn’t even stop when a disk fails. You connect up a new disk live to the computer (which you can do with some systems) and it is recreated from the working disks, and you never miss a beat. This is pretty important with a major server.

But RAID has value to those who are not in the 99.99% uptime community. Those who are not good at doing manual backups, but who want to be protected from the inevitable disk failures. Today it is hard to set up, or expensive, or both. There are some external boxes like the “readynas” that make it reasonably easy for external disks, but they don’t have the bandwidth to be your full time disks.

RAID-5 on old IDE systems was hard, they usually could truly talk to only 2 disks at a time. The new SATA bus is much better, as many motherboards have 4 connectors, though soon one will be required by blu-ray drives.  read more »