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I've seen both of these listed, both being striped and mirrored across multiple drives, but is the a difference between them that I'm not picking up on?

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migrated from stackoverflow.com May 26 '10 at 2:39

2 Answers

up vote 24 down vote

It has to do with the order that the operations are performed in, and it only appies to arrays that are 6 disks or larger (if you have 4 disks, they're both pretty much the same).

RAID 1+0 (10): Disks 1 + 2, 3 + 4, 5 + 6 are mirrored to create a RAID-1 array, and a RAID 0 array is created ontop of the arrays.

RAID 0+1 (01): Disk 1 + 2 + 3 are striped to create a RAID 0 array, and then disks 4 + 5 + 6 to create RAID 1 redundancy.

With RAID 0+1, a single disk loss from one side of the array (1,2,3 or 4,5,6) will degrade the array to a state where you are essentially running RAID 0 (which is bad).

With RAID 1+0, you can lose a single disk from each pair (1,2 or 3,4 or 5,6) and the array will stay functional. The only way this array can be brought offline is to have both disks in a pair fail.

Unless your circumstances are exceptional, you should never use 0+1.

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+1 avoid 0+1 it should only be used in very special case – radius May 26 '10 at 3:30
Care to enlighten us on one such "exceptional" case for 0+1? I'm curious :) – Earlz May 26 '10 at 6:51
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I can't think of any, hence why if you did have one, it would be exceptional – Mark Henderson♦ May 26 '10 at 7:06
If you're using a cheap card, check the documentation to be sure of the order. I've seen cheap cards that say "RAID 10" and actually implement "RAID 01" – Chris S♦ May 26 '10 at 13:31
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Just one quick note, in either case the array is considered degraded after the loss of one disk, and that disk will need to be replaced ASAP as the loss of one more disk can result in data loss. The chances are 1/7 for RAID1+0, and 4/7 for RAID0+1. But in either case, 1 more failure can take down the whole array – Andrew Lowe Jul 12 '10 at 9:18
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This belongs on ServerFault but here is a quick overview of the difference from Wikipedia

RAID 10

RAID 1+0 (or 10) is a mirrored data set (RAID 1) which is then striped (RAID 0), hence the "1+0" name. A RAID 1+0 array requires a minimum of four drives – two mirrored drives to hold half of the striped data, plus another two mirrored for the other half of the data. In Linux, MD RAID 10 is a non-nested RAID type like RAID 1 that only requires a minimum of two drives and may give read performance on the level of RAID 0.

RAID 01

RAID 0+1 (or 01) is a striped data set (RAID 0) which is then mirrored (RAID 1). A RAID 0+1 array requires a minimum of four drives: two to hold the striped data, plus another two to mirror the first pair.

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