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So last year I helped my father in France set up his Office machine with a RAID5 array of 3 identical 1TB drives (for a combined space of 2TB). Last week one of the drives died, he's using the machine in degraded mode but I'd like to get that sorted ASAP.

The thing is after he got a replacement drive he put it in and the system doesn't recognise there is a physical drive there. After some testing with cables and both the original drive and the replacement we've come to the conclusion that the motherboard has stopped recognising that connection.

The system currently has a small drive it boots from and the raid array is then mounted on /home. It's got 4 SATA slots, 3 taken up by the boot drive and the two working parts of the array and the fourth being the one that's died.

Without replacing the motherboard and having to take into account that I'm in England and he's in France so I can't just pop round and try stuff out my thought is to fall back to RAID0. He loses the redundancy but keeps the same amount of space which to my mind means it should be possible (moving to a 1TB RAID1 array seems out of question).

But I've generally not had to deal with such matters, I'm more of a developer, can someone tell me if it's possible to go from RAID5 to RAID0 in place or will I need to wipe it, rebuild it and the copy the data back from a backup?

Thanks. I hope there's enough information here.

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According to the section Grow Mode in man mdadm, it is possible, but you need space for a backup file. I couldn't find any information on how big the backup file needs to be. –  Dennis Mar 15 '13 at 16:25
    
To be honest, a degraded RAID5 array (like what you have now) is mathematically comparable to a RAID0 (stripe). You lack the redundancy, and the data is striped. The giant drawback is that you have to compute checksums, so performance is lower than a stripe. –  Mutant Bob Mar 7 at 16:22
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2 Answers 2

While it is mathematically possible to perform a procedure where you convert a RAID5 to a RAID0, I don't think any implementations have bothered to code this.

To be honest, a degraded RAID5 array (like what you have now) is mathematically comparable to a RAID0 (stripe). You lack the redundancy, and the data is striped. The giant drawback is that you have to compute checksums, so performance is lower than a stripe. When reading large files it's probably a negligible hit. When reading small files, the hit will be harder, but it's a small file so you probably won't notice it.

My recommendation is that you get a PCI SATA card and install that to control the new drive. When I filled up all 6 SATA connections on my motherboard, I just dusted off an ancient Promise TX4 I had lying around. You can get better cards cheaper these days.

I do agree with Ramhound about impending failure. You want to get that RAID reconstructed ASAP and budget for the death of the other drives in the next 1-20 months.

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I hope you mean RAID 1 rather than RAID 0. RAID 0 is striping and provides no fault tolerance. However, regardless of what type of RAID you want to switch to you will need to copy the data off of the existing RAID and rebuild it. I know of no way to switch a RAID in place.

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I unfortunately meant RAID0 as this would mean the size of the array would be constant. I figured this would mean it could be possible. If we have to go with a full wipe and rebuild I guess we'll have to. –  Scimon Mar 15 '13 at 15:56
    
You would be greatly increasing your risk of data loss. If one of the drives fail the entire RAID would be unreadable. –  Brad Patton Mar 15 '13 at 15:59
    
If one drive is failing its only a matter of time before the rest goes. –  Ramhound Mar 15 '13 at 17:35
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