How to use RAID in 2014

Years ago I wrote a post "Why RAID 5 stops working in 2009". Five years later you can still buy many RAID5 arrays. Was I wrong?

No. In that post I laid out the simple problem with RAID 5 in 2009:

  • Disks fail.
  • When they fail the remaining disks are asked to read every available sector to rebuild the lost data on a new drive. 
  • The common unrecoverable read error (URE) spec is often still 1 in every 10^14 bits read. One error in about every 12.5TB.
  • With a 4 drive RAID 5 using 4TB desktop drives, that's 12TB of capacity - and a high likelihood of an URE.
  • When the URE happens your rebuild fails.

Now what?
First, don't panic. I own 3 different 4 drive arrays and I'm OK.


Because I understand the limitations of the technology and take steps to protect my data. What is the key step?

Don't keep your only copy of important data on a single array!

I keep data copied between arrays - a simple form of mirroring - so if an array fails I still have my data AND can recreate it if a drive rebuild fails.

Limits of RAID 5
With today's large drives you need to look at their error rates. Desktop drives are typically cheaper and have a 10^14 spec.

But drives intended for RAID typically have a higher spec: 10^15, which is one URE every 100GB or so. Much safer.

But regardless of the drive spec, you always need to back up! Whether to another array, as I do, or to the cloud or tape, you cannot rely on any storage device to safely keep your only copy of important data.

The Storage Bits take
RAID 5 still "works" in 2014. But only if you take precautions.

I speak to end-users who thought RAID meant their data was safe - and then discover it wasn't.

RAID offers speed and capacity, but safety comes from copies, not RAID. Remember that and you'll be fine - even in 2024.l

Comments welcome, as always. What is your RAID backup strategy?