RAID 5 protects against a single disk failure. You can recover all your data if a single disk breaks. The problem: once a disk breaks, there is another increasingly common failure lurking. And in 2009 it is highly certain it will find you.

Disks fail While disks are incredibly reliable devices, they do fail. Our best data - from CMU and Google - finds that over 3% of drives fail each year in the first three years of drive life, and then failure rates start rising fast.

With 7 brand new disks, you have ~20% chance of seeing a disk failure each year. Factor in the rising failure rate with age and over 4 years you are almost certain to see a disk failure during the life of those disks.

But you're protected by RAID 5, right? Not in 2009.

Reads fail SATA drives are commonly specified with an unrecoverable read error rate (URE) of 10^14. Which means that once every 100,000,000,000,000 bits, the disk will very politely tell you that, so sorry, but I really, truly can't read that sector back to you.

One hundred trillion bits is about 12 terabytes. Sound like a lot? Not in 2009. //

So now what? The obvious answer, and the one that storage marketers have begun trumpeting, is RAID 6, which protects your data against 2 failures. Which is all well and good, until you consider this: as drives increase in size, any drive failure will always be accompanied by a read error. So RAID 6 will give you no more protection than RAID 5 does now, but you'll pay more anyway for extra disk capacity and slower write performance.

Gee, paying more for less! I can hardly wait! //

Originally the developers of RAID suggested RAID 6 as a means of protecting against 2 disk failures. As we now know, a single disk failure means a second disk failure is much more likely - see the CMU pdf Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You? for details - or check out my synopsis in Everything You Know About Disks Is Wrong. RAID 5 protection is a little dodgy today due to this effect and RAID 6 - in a few years - won't be able to help.

Finally, I recalculated the AFR for 7 drives using the 3.1% AFR from the CMU paper, using the formula suggested by a couple of readers - 1-96.9 ^# of disks - and got 19.8%. So I changed the ~23% number to ~20%.

Comments welcome, of course. I revisited this piece in 2013 in Has RAID5 stopped working? Now that we have 6TB drives - some with the same 10^14 URE - the problem is worse than ever.