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I had a massive data lost with my last raid5 (with 1 spare out of 4 hdds) using mdadm, because 2 drives failed within 1 day (unfortunately).

Now I bought new hdds and now have a total of 6 hdds. I want to build a new raid which can handle the loss of two drives, independant of which 2 are failing.

HW-Raid and LVM is no option in my environment.

I had an mdadm raid10 in mind (but have no idea how do that practically): - 6 devices in common - 3 spares. So I'd have a loss of 50% of space but could handle 2 faults

May anyone has an idea how to do that or another approach?

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4 Answers 4

up vote 4 down vote accepted

If you want to be able to tolerate two HD drives failures, you can use Raid6 on all 6 drives.

Using Raid5 with hot-spare does not protect from two consecutive failures because the 2nd failure may happen before completing the rebuild process.

The following image taken from Wikipedia shows how Raid6 works. You can see that two HD drives are used for parity for each block of data which are denoted as A[p], A[q], B[p], B[q] and so on.

enter image description here

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@SvenW: I already said that clearly! –  Khaled Jun 14 '12 at 9:01
    
@SvenW: Maybe, you are right. My statement is not completely correct. I fixed it. Thanks! –  Khaled Jun 14 '12 at 9:05
    
OK, great :) retracted the d/v –  Sven Jun 14 '12 at 9:08
    
Thx a lot for your anwers. Did I understood correctly that it tolerates two fails, but I can't set it up to tolerate under certain circumstances 3 fails, right? (Which was not questioned above) –  McK Jun 14 '12 at 11:44
    
@Mck: Raid6 supports two HD failures which should be fine for your question. You can add one more HD as hot spare for added reliability. –  Khaled Jun 14 '12 at 11:47

This depends on your storage needs regarding size and throughput, but a RAID6 appears to be the best solution.

If you can handle the lost space, I would even consider a spare drive in this situation as well.

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RAID10 is ALMOST "2 drives safe" but some pathological combinations of two dead drives can take it out. So RAID6 it is.

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Isn't it possible to have 3 drives as an raid1 and another 3 with raid1, so that you can have a raid 0 over these two trios? –  McK Jun 14 '12 at 11:46
    
@McK with many implementations, yes. You can do that if you're super paranoid. –  MDMarra Jun 14 '12 at 11:55

Also an good idea would be to keep an offsite backup. If you data is very valuable.

Providers: - Crashplan - Amazon S3 - Many more...

I use Crashplan myself. Backup my important files from an RAID5 array.

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