Re: What RAID type and why?

On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow [off-list ref] wrote:

Neil Brown [off-list ref] writes:

quoted

On Sat, 06 Mar 2010 18:17:44 -0500
"Guy Watkins" [off-list ref] wrote:

quoted

}
} At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
} which may be a problem.

If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
and you don't have the RAID6 I/O overhead.

and as md/raid6 requires at least 4 drives, RAID1 is not just the best
solution to survive two failures on a 3-device array, it is the only solution.

NeilBrown

Except that there also is raid10 with 3 mirrors. :)

MfG
       Goswin

PS: Why doesn't raid6 still not allow 3 drives for the special case of
converting raid1 -> raid6?


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That should be obvious:

Possible stripes:

Start:
1, 1, 1;
2, 2, 2;

'raid6' overtake...
1, q, Q;
2, q, Q;

'raid6' overtake with missing;
1, (missing 2), q, Q;
3, (missing 4), q, Q;

In the first overtake case you have the requirement of generating 200%
parity, which probably won't work for the algorithm and is a silly
idea in general since it's computationally far less expensive to store
another copy of either form of data instead.

In the second you're gaining the space of a second disk at the cost of
being already degraded; why not just go for raid 5 instead?

You can overtake raid5 later with raid6 if you add more devices.
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