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Nested RAID 0+0 - ultimate performance?

Last response: in Storage
January 6, 2011 12:52:16 PM

Nested RAID levels are of course nothing new - RAID 1+0 and RAID 0+1 being the most common, giving you advantages of both mirroring and striping for both redundancy and performance.

But what if you nested RAID 0 twice? In other words, four disks, each pair a RAID 0 array, and then those arrays also in RAID 0?

RAID 0 gives you almost double the performance of a single disk (much as SLI gives you almost double the performance of a single GPU), at double the cost (double the drives). Does nesting RAID 0 scale linearly? Would RAID 0+0 give you almost 4x performance at 4x cost?

Triple SLI doesn't quite give you triple performance, as there is some overhead in coordinating between the cards, In the case of RAID, the overhead is borne by the RAID controllers, however, and theoretically each controller only has to worry about 2 logical units. So I would expect that nesting level 0 RAID arrays would be less burdened by overhead and would be closer to true linear scaling.

Has anyone ever done this? It's insanely expensive of course - 4 disks, with 4x more risk of drive failure and absolutely no redundancy at all. Though you could envision a RAID 0+0+1 array where you have 4 disks in RAID 0+0 and then do a simple RAID 1 array at the very top with a much larger drive. An example would be to do RAID 0+0+1 with 4 128 GB SSDs and 1 500 GB hard disk. It would be easy to simply reduce the nesting level for performance comparisons, to see how RAID 0+0+1 fares against RAID 0+1, RAID 1+0, RAID 0, and RAID 1 as the baseline.

I don't have 4 SSDs and a spare 500 GB disk lying around. Or 5 hard drives of any sort, frankly. But I bet the Tom's folks have the hardware to spare lying around the bench. If this topic intrigues you, then reply/mod/fan this thread and let's see if we can get the Editors' attention!

(This discussion of RAID 0+0 is cross-posted at my geek blog, where I've been hardware build chronicling my new enthusiast system for World of Warcraft and MATLAB)
a b G Storage
January 6, 2011 4:41:56 PM

I don't think you can do the 0 0 1 type stuff with different size drives that you are talking about.

It's not nested RAID. You can make a multi-volume RAID 0 array with 3 or 4 drives. You will get better transfer rates, as you mentioned at reduced reliability. I don't know if it's purely linear. It would depend on the workload as well.
January 6, 2011 5:28:27 PM

keep inmind that many "performance" SSd drives are actually RAID 0 inside already, transparent to the user. If you did RAID 0 with such disks, you're effectively already doing RAID 0+0.

I think that nested RAID 0+0 has less overhead than doing RAID 0 with 4 disks, because one controller has to deal with four logical units in the latter case, whereas if you do RAID 0+0 you have three RAID controllers, each only doing overhead for two logical units. This is why i think the linearity of scaling would be better with a RAID 0+0 setup.

a b G Storage
January 6, 2011 10:52:45 PM

Are you talking hardware or software RAID?

If you are talking hardware RAID, then generally all of the disks will need to be attached to the controller that is doing the RAID.

If you are talking software RAID, then the CPU and the OS are doing the work, and it doesn't matter how many different controllers you are attached to.