[ale] new (to me) raid 4/5 failure mode

[Greg Freemyer]

All,

If you are using raid 4 or 5 or considering it in unreliable
environments, you may want to think about this.  By unreliable I mean
your system fails at unpredictable times due to power, bad hardware,
kernel crashes, etc.

Anyway, here's the failure mode:

In normal 3-disk operation:  p = d1 ^ d2, thus we have redundancy.

If d1 dies the array continues to run in degraded mode:
  d2 ^ p ==> d1

Now if you write data to d2, p gets updated as well:  so

d2' ^ p' ==> d1   (note the primes indicate new data values)

As expected d1 can still be recreated from d2' and p'.

But in general d2 and p updates are non-atomic in relation to each
other, so their is a short period time where either:

d2' ^ p ==> garbage or
d2 ^ p' ==> garbage

So if a system or power fail occurs, d1 becomes garbage, even though
it was never written to by an application!

Obviously, I don't mean the entire disk.  Just those d1 chunks that
are part of a partially updated stripe.

I had never considered that stable data sitting on a raid 5 might
change randomly, even if they were never written to.  I have never
been a fan of raid 5.  In fact, I only considered it a good choice for
low-end situations.  I think the above rules it out for most of those.

I think raid 6 would only have a similar issue in a dual disk failure
mode, but I'm not positive.

Greg
-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
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