On Wed, Mar 10, 2010 at 9:47 AM, Goswin von Brederlow [off-list ref] wrote:

Michael Evans [off-list ref] writes:

quoted

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

quoted

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;

Start:
1, 1, 1;
2, 2, 2;
3, 3, 3;
...

quoted

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

Middle:
1, P, Q;
P, Q, 2;
Q, 3, P;
...

End:
1, 2, P, Q;
4, P, Q, 3;
P, Q, 5, 6;
...

quoted

'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.

The sick 3 disk raid6 case should have both the P and Q identical to the
data block. It is indeed computational a waste to go through the
expensive P/Q parity algorithm for the same result as mirroring but this
is only ment as a transitional state.

quoted

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.

Because then you are going from 2 mirror disks to 1 parity disk even if
only temporary. You are reducing the number of disks failures you can
survive from 2 to 1 and the high load during a reshape makes a failure
more likely than normal operations.

Or can you go from 3 way raid1 to 4 disk raid6 in a single step?

MfG
       Goswin

You are not planning on staying with 3 devices though.

Just stick with 2 redundancy raid 1 until you have four devices.
Then overtaking from raid 1 + hotspares at 4 devices total to raid 6
with 2 data devices and 2 parity devices per stripe makes sense.
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