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Re: What is the point of RAID?



lee wrote:
On Tue, 11 Nov 2008 19:54:11 -0600
Mark Allums <mark@allums.com> wrote:

that three drives is 50% more likely to fail than two.  More than
fifty percent, if I remember my statistics at all correctly.

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?

Uh, I am trying to remember the binomial theorem from statistics. Nothing to do specifically with HDDs, just applied mathmatics.



If you
have a RAID 50 running on 20 SAS drives and 4 hot spares, you better
buy quite a few for cold spares, you are going to lose a drive every
two months. At least.

You are saying that the age of the drives doesn't matter at all? Then if
you lose one drive out of 24 every month, that would mean that about 4%
of all drives sold are junk. The new ones you get could fail within the
first few minutes ... or not work at all. Or does this mean that it
takes about one to two months before you find out if a new drive is
junk? And why don't the drives that are junk fail in the first few
minutes or don't don't work at all?


Age matters. Drive either fail in the first 60 days, or last for the full length of the design life. Except when they don't.

Experience teaches that accidents happen. The Boy Scout motto is: Be prepared. That's all I'm saying.

Mark Allums



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