Does RAID 0+1 instead of RAID 10. Really? Seriously?
I was contemplating ordering one of these with a Mac Pro because I wanted to use it in RAID 10. RAID 10 is a stripe (RAID 0) across two or more mirrored pairs (RAID 1). Conversely, RAID 0+1 is a mirror across two or more striped pairs. But as the manual clearly states, it doesn’t do RAID 10. It does RAID 0+1, and that’s not so good.
There are dramatic differences between these two configurations:
- Chances of array failure:
“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 number of drives in the system. The chance of system failure in a RAID 1+0 system with two 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.”
- Performance in degraded state:
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 the right 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 write performance 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 degraded mode than RAID 0+1.
- Recovery once a disk is replaced:
RAID 1+0 only has to re-mirror one drive, whereas RAID 0+1 has to re-mirror the entire failed set. So RAID 1+0 will recover significantly faster.
Given these pretty significant failings of RAID 0+1 as compared to RAID 10, it’s a pretty good question as to why Apple would choose the much poorer option for its ‘Pro’ RAID card, for which it charges $720. I’m just really left scratching my head.
Shame on you Apple, shame!