Re: What is the point of RAID?
On Wed, Nov 12, 2008 at 08:53:46AM -0500, Jeff Soules wrote:
> On Wed, Nov 12, 2008 at 3:44 AM, lee <lee@yun.yagibdah.de> wrote:
> > Do you mean it is more likely that any one drive in the array fails when
> > you have more drives, or do you mean that it is more likely for a drive
> > in the array to fail when you have more drives? If drives fail more
> > often when being used in an array with more drives, what makes them
> > fail more often under those conditions?
>
> It's purely a statistical property, not related to being in a RAID
> array. But if there's (say) a 5% chance for a given drive to fail on
> a given day, there's a 95% chance it won't fail.
> If you have two drives, the chance *both* won't fail is the chance of
> one not failing, times the chance of the other not failing -- 95%
> times 95%, or 90.25%.
>
> With 24, the chance of all the drives not failing is .95^24 or 29.2%.
>
> Of course I just made the rates up, the survival chances of individual
> drives are higher. But logic holds; the more drives you're watching,
> the more lucky you'd have to be for none of them to be a dud.
>
Jeff,
you math is off - way off.
P(one fails) != 5/100
P(two drives fail at the same time) = P(one fails) * P(one fails)
= 25/10000
If you have more than 2 drives in the raid you have to make the
cobinatoric calculations of how many configuration can be there for two
drives out of n.
that would be
2! * (n-2)! / n!
multiply that to P( two drives fail at the same time) where n is the
number of all drives.
Henning
--
Henning Follmann | hfollmann@itcfollmann.com
it consultant | www.itcfollmann.com
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