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RAID question.

P

piako

[H]ard|Gawd
Joined
Apr 5, 2006
Messages
1,613
How often does a RAID-5 array (small size ~6 disks) fail during rebuild (another disk dies during rebuild = total data loss)?
 
piako said:
How often does a RAID-5 array (small size ~6 disks) fail during rebuild (another disk dies during rebuild = total data loss)?
Click to expand...

If you assume that the chance of every individual disk is 33% per year, then the chance of fail is 4.5716763e-5 per hour.

If your rebuild takes 5 hours, then the chance (per disk) is 2.2858381e-4.

If you have an array with 6 disks live + one hot spare (or one inserted right after failure), then there are 5 live disks, but the new spare can fail, too, during those 5 hours, so your risk applies to 6 disks.

That puts the total risk at 0.001370728 or roughly 1/1000.

%%

The problem is, of course, that disks often break not out of random chance, but that some external event damaged them, such as:
- power fluctuations in A/C support
- power fluctuations due to broken PSU
- temperature fluctuations
- fan failure and overheat
- somebody kicks the case
- other causes of mechanical impact such as loose screw on disk cage or earthquake

Then the chance of a second drive failure right after the first is several orders of magnitude higher.

But of course, if that happens the chance of a triple failure is also high, which makes a three-disk raid-0 or a raid-6 almost equally useless.
 
ty uOpt, those probabilities seem pretty low. Care to share the equations?
 
piako said:
ty uOpt, those probabilities seem pretty low. Care to share the equations?
Click to expand...

This is normal probably multiplication. I didn't keep the cut'n'paste from the above run.

Here's it backwards for proof, single disk over 5 hours converted to pre-year chance of failure:
Code: 
(defun prob* (prob n)
  (- 1.0 (expt (- 1.0 prob) n)))

:user - Yes, Master? (cra:prob* 2.2858381e-4 (/ (* 24 365) 5))
0.3300315
:user - Yes, Master?
 
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