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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?
This question already has an answer here: 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? 

migrated from stackoverflow.com May 26 '10 at 2:39This question came from our site for professional and enthusiast programmers. marked as duplicate by MDMarra, MadHatter, Jenny D, Scott Pack, Ward Nov 5 '13 at 18:43This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question. 


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 nonnested 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. 


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 RAID1 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. 


Raid 0+1 vs Raid 1+0 (Probability of Failure)Here's a little bit of math that should show the differences in rates of failure. For simplicity, let's assume there is an even number of disks. In both array configurations, each disk is broken up into blocks. In Raid 0+1, striping occurs first and then mirroring. In Raid 1+0, mirroring occurs first and then striping. We can always partition Raid 0+1 into two groups (G1 and G2).
In either Raid configuration, at least two disks must die. Let's look at all the possible ways both Raid configurations could fail if two disks were to die.
With 4 disks, there are C(n, 2) = C(4, 2) = 6 combinations in total. ^{4}/_{6} of these combinations would cause a Raid 0+1 configuration to fail. (66% chance of failure)
With 6 disks, there are c(n, 2) = c(6, 2) = 15 possible combinations. Now these results can be generalized for n disks.
Now the most useful and interesting part of the math. We can take the limits of the two equations above. Below, I use 'inf' to mean infinity.
In other words, there will always be at least a 50% chance of failure if 2 disks die on a Raid 0+1 configuration! Now let's see how a Raid 1+0 configuration fairs.
In other words, the more disks we add to a raid 1+0 configuration, the closer to a theoretical 0% chance of failure we get! One final table (Please note that I am rounding the values to integers.)
Conclusion: Use Raid 1+0. 

