Re:

[David Greaves <david@dgreaves.com>]

Ross Vandegrift wrote:

>On Thu, Jan 12, 2006 at 11:16:36AM +0000, David Greaves wrote:
>  
>
>>ok, first off: a 14 device raid1 is 14 times more likely to lose *all*
>>your data than a single device.
>>    
>>
>
>No, this is completely incorrect.  Let A denote the event that a single
>disk has failed, A_i denote the event that i disks have failed.
>Suppose P(A) = x.  Then by Bayes's Law the probability that an n disk RAID
>will lose all of your data is:
>
>n_1 = P(A) = x
>n_2 = P(A_2) = P(A) * P(A_1 | A) = x^2
>n_3 = P(A_3) = P(A) * P(A_2 | A) = x^3
>...
>n_i = P(A_i) = P(A) * P(A_{i-1} | A) = x^i
>
>ie, RAID1 is expoentially more reliable as you add extra disks!
>
>This assumes that disk failures are independant - ie, that you
>correctly configure disks (don't use master and slave on an IDE
>channel!), and replace failed disks as soon as they fail.
>
>This is why adding more disks to a RAID1 is rare - x^2 is going to be
>a really low probability!  It will be far, far more common for
>operator error to break a RAID than for both devices to honestly fail.
>
>  
>
sorry, read it all as 'linear', not mirrored which is why I was writing
drivel ;)

David


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