8/9/2019 Why is Raid 1+0 Better Than Raid 0+1

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Why is RAID 1+0 better than RAID 0+1?

RAID 0+ 1

RAID 0+1 configuration where multiple disks are striped together intosets (sets A & B in the diagram, each set being as large as the resulting

final volume), and then two or more sets are mirrored together.

RAID 1+ 0

RAID 1+0 configuration where two or more drives are mirrored together

(mirrors 1-4 in the diagram), and then the mirrors (as many as are needed

to result in the desired amount of space) are striped together.

In either case (0+1 or 1+0), the loss of a single drive does not result in failure of theRAID system. The difference comes in the chance that the loss of a second drive from the

system will result in the failure of the whole system. In RAID 0+1, you have to lose onedrive from each disk set to result in the failure of the whole system. In my diagram thatwould be one drive from set A and one drive from set B. In RAID 1+0, you have to lose

all drives in a mirror. This would be both drives in any numbered pair in the diagram.

Mathematically, the difference is that the chance of system failure with two drive failures

in a RAID 0+1 system with two sets of drives is (n/2)/(n - 1) where n is the total numberof drives in the system. The chance of system failure in a RAID 1+0 system with two

8/9/2019 Why is Raid 1+0 Better Than Raid 0+1

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drives per mirror is 1/(n - 1). So, using the 8 drive systems shown in the diagrams, the

chance that losing a second drive would bring down the RAID system is 4/7 with a RAID

0+1 system and 1/7 with a RAID 1+0 system.

The math gets more complicated when you have more than two elements to a mirror.

Since that's a rare configuration, Same has been figured out in equations

Another difference between the two RAID configurations is performance when the

system is in a degraded state, i.e. after it has lost one or more drives but has not lost theright combination of drives to completely fail. In a RAID 0+1 configuration, the loss of

any drive in a set causes the failure of that entire set and the set is removed from the

RAID system. Generally (in the two set case) this means you are left with a RAID 0

system made up of the remaining set of disks. This probably slightly improves writeperformance and slightly degrades read performance (but that's just a WAG, I haven't

done any testing). In a RAID 1+0 system, you would see the same effect on each mirror

that loses a drive, but not the whole system. In other words, a RAID 1+0 configuration

will tend to show similar, but less dramatic, changes in performance when in a degradedmode than RAID 0+1. However, the changes will likely be slight in any case.

One more difference that was recently pointed out to me is the speed at which the RAID

system recovers once the failed disk is replaced. RAID 1+0 only has to re-mirror onedrive, whereas RAID 0+1 has to re-mirror the entire failed set. So RAID 1+0 will recover

significantly faster.