It’s always kind of a crap shoot to compare performance in real world scenarios. What I mean is whatever numbers you get will be different than mine. Your Drobo will be loaded with a different mix of data at that 60% mark than mine will. I’m using HFS+ and you (might) be using NTFS, for example. The value in the number is in the ballpark figure.
Where I have compared numbers is between my Drobo, my QNAP and a directly attached FW800 drive on the same computer. All were 40 to 60% full and the results told me that my FW800 bus is fine (the Seagate FreeAgent 1TB clocked in respectable speeds of around 60MB/s versus Drobo’s 24MB/s when both were the only device on the bus, both at 800Mb/s). The QNAP was for a benchmark since it’s accessible via IP only and it beat the Seagate FreeAgent handily.
I’ve never really understood why BeyondRAID is so bloody slow and lost interest in figuring it out. I think it’s a combination of CPU and the algorithm itself but that’s just a guess. For example, I swapped two drives recently, one after the other. This took 5 days, 2 for the first and 3 for the second. Drobo has 2.1TB of data and went from 2.7TB to 3.6TB (which is another oddity I’ll get to in a minute) by swapping 2 1TB drives for 2 2TB drives. When that finished I shut Drobo down as I was leaving on a business trip. This week he’s been online and the drives are chunking away, I suspect moving, re-mapping and optimizing blocks after the change. All week.
In contrast, when I do the same with my QNAP the re-build of parity takes about a day for each drive swapped/replaced and then the re-lay about two days, so figure five days tops for two drives. Drobo has been “at it” for 10 so far.
Regarding the drive mix, here’s some funny math. When I had 6TB in Drobo in the form of 4 1.5TB drives I had 4.1TB of usable space. Now that I have 6TB in the form of 2 2TB and 2 1TB I have 3.6TB of usable space with an extra 500GB going for “protection”. Drobolator confirms this so it’s no secret, just something I find odd. And yes in RAID5 I know that I’d have to swap all four drives to get any additional storage.