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Recovering from a two-drive failure in a RAID 10 configuration

In a RAID 10 configuration with four drives, can you recover data if two drives fail?

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A RAID 10 configuration comprises two or more mirrored pairs of drives. Drives that are on each "side" of the mirror have data striped across them in a RAID 0 format. This combination of mirrored pairs (RAID 1) and striping (RAID 0) is how RAID 10 gets its name. Because you must have at least two mirrored pairs to create a RAID 10 array, the minimum number of drives in a RAID 10 format has to be four.

So it's possible to recover data if two drives in a RAID 10 configuration fail, but it's dependent upon which two drives fail. Consider a four-drive RAID 10 setup where drives A and B have data striped across them and are mirrored by drives C and D on the opposite side. This RAID group can recover if any or all drives in a stripe fail, but not if both drives in a particular mirror fail. Therefore, we can recover if drives A and B or drives C and D fail, but not if drives A and C or drives B and D fail.

This was first published in September 2009


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