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Re: What is the point of RAID?



lee wrote:
On Wed, 12 Nov 2008 09:59:09 -0600
"Boyd Stephen Smith Jr." <bss03@volumehost.net> wrote:

So what is the optimal number of disks in a raid 5 and a raid 1?
If by optimal, you mean, least chance of failure:

Not exactly; I was wondering if there is a breaking point, as in
"adding more drives only increases the chances of the whole array to
fail" beyond that point, and "adding more drives reduces the chances of
the whole array failing" before that point.

Wouldn't it be useful if that breaking point was known for all kinds of
raid setups? The calculation would have to consider the chances of
several drives failing at (about) the same time.

It is known for quite a few. A RAID 5 is possible for a huge number of drives, and double that number for RAID 50. RAID 6 allows more drives to fail at once, but comes with a cost. About 6 or 7 drives is the practical limit for 5. There comes a point when adding drives *decreases* the overall reliability. And performance, as well.

Mark Allums



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