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I have an HP Smart Array P400i raid controller, with 6 disks, working in a RAID 1+0. This is what it looks like:

     Mirror Group 0:
        physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SAS, 72 GB, OK)
        physicaldrive 1I:1:2 (port 1I:box 1:bay 2, SAS, 72 GB, OK)
        physicaldrive 1I:1:3 (port 1I:box 1:bay 3, SAS, 72 GB, OK)
     Mirror Group 1:
        physicaldrive 1I:1:4 (port 1I:box 1:bay 4, SAS, 72 GB, OK)
        physicaldrive 2I:1:5 (port 2I:box 1:bay 5, SAS, 72 GB, OK)
        physicaldrive 2I:1:6 (port 2I:box 1:bay 6, SAS, 72 GB, OK)

If my understanding is correct, drives 1 and 2 could fail, and the RAID would still be ok.

This doesn't seem right to me, I was thinking a RAID 1+0, with 6 drives should look like this:

     Mirror Group 0:
        physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SAS, 72 GB, OK)
        physicaldrive 1I:1:2 (port 1I:box 1:bay 2, SAS, 72 GB, OK)
     Mirror Group 1:
        physicaldrive 1I:1:3 (port 1I:box 1:bay 3, SAS, 72 GB, OK)
        physicaldrive 1I:1:4 (port 1I:box 1:bay 4, SAS, 72 GB, OK)
     Mirror Group 2:
        physicaldrive 2I:1:5 (port 2I:box 1:bay 5, SAS, 72 GB, OK)
        physicaldrive 2I:1:6 (port 2I:box 1:bay 6, SAS, 72 GB, OK)

The data would be striped across the 3 mirrored pairs.

Are both configurations possible, or just the first?

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2 Answers 2

up vote 2 down vote accepted

You do not have a choice on the drive arrangement/pairs.

In that arrangement HP Smart Array controllers split the disks as such:

physicaldrive 1I:1:1 pairs to physicaldrive 1I:1:4 
physicaldrive 1I:1:2 pairs to physicaldrive 2I:1:5
physicaldrive 1I:1:3 pairs to physicaldrive 2I:1:6 

RAID 1+0 == RAID 1 pairs, striped (RAID 0) together...

You can lose ONE disk from each of the pairings listed above. Or, one drive from each "Mirror Group".

Also: What are the different widely used RAID levels and when should I consider them?

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1  
Shouldn't you be able to lose every single disk from a mirror group as long as the other mirror group is intact or am I missing something about the HP-specific terminology here. –  MDMarra Apr 20 '12 at 16:43
    
Yes, you can lose all of the disks in single mirror group as long as the other mirror group is healthy. In HP terminology, the Mirror Group 0 and 1 just represent the division of one side of the mirror from the other. –  ewwhite Apr 20 '12 at 16:46
2  
The output from the controller seems counter intuitive to me. But you're right. I killed drive 1, 2, and 3, and everything is still fine. So 1 is a mirror of 4, 2 is a mirror of 5, and 3 is a mirror of 6. That's the setup I want. –  Matt Apr 20 '12 at 16:50

Both configurations are probably possible but using the first seems riskier.

The first situation leaves you with, depending on implementation in the controller, possibly a longer rebuild (it may have to read all three drives of the stripe to rebuild the mirror made of three drives) as well as if you lose a drive out of each stripe set the array fails.

The second provides faster rebuild (only rebuild a single drive to its mirror) as well as better protection 1 drive of each mirror can fail before the array is no longer available.

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This is still RAID 1+0. The HP controller pairs a drive from one Mirror Group to a drive from the other Mirror Group. –  ewwhite Apr 20 '12 at 16:22

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