Raid 5

Corey Edwards tensai at zmonkey.org
Fri Dec 2 10:17:46 MST 2005


On Fri, 2005-12-02 at 09:55 -0700, Steve Meyers wrote:
> Corey Edwards wrote:
> > Now with RAID 10 you lose capacity vs the same disks in a RAID 5, but as
> > you add more disks the chance that two drives in the same RAID 1 pair go
> > out actually goes down. And in the performance category, you've got more
> > spindles.
> 
> So you're telling me that if I have two disks in a RAID 1 pair, and then
> I add two more disks in a RAID 1 pair, the chances of both disks in the
> first pair dying at the same time go down?  I don't buy that.  They're
> completely independent of the second pair.  Because they're independent,
> you actually add them together (kind of), and you increase your chances
> of a pair failing.

OK, that is what I said but upon reflection that's not what I was
actually thinking of. What I was getting at is that *any* two disks in a
RAID 5 fail and you're hosed. With a RAID 10 you need a certain
combination of disks to fail. My statistics is way too rusty to make a
stab at the numbers, but even I can see that a RAID 10 is a lot less
risky. I would be interested in seeing the calculations if anybody out
there is up for it.

Corey

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