I'll second this - I've done a lot of raid set rebuilds, and had a few double failures.
So for critical systems (databases, etc.), running with enough redundancy to survive two drive failures is a great idea. My guess (and I was in the hard drive business for a few years) is that the rebuild process puts a high load of stress on the rest of the drives, making it more likely that another drive fails during the rebuild. On top of that, if all of the drives are the same model and age they are more likely to fail for the same reasons at around the same time, if there's a mechanical cause of failure. Even if that's not the case, rebuild times for large drive arrays built out of big, slow disks can be *days* during which you're running with no redundancy, which is dangerous.
Drobo supports that as an option, which they call "Dual Disk Redundancy". If you have one of their larger units (e.g. the 8 bay Drobo Pro) you're only spending two drives to protect 6, which seems like a pretty reasonable overhead to secure your data. In a smaller, four drive unit, you'd be using two drives to protect two drives, which would work, technically, but seems inefficient since you're wasting a lot of disk space.
On Thu, Nov 8, 2012 at 5:18 PM, paul <[address removed]>
How many RIAD5 systems do you operate? In my experience with dozens of RAID5, 6 and 10 subsystems, when a drive fails on a subsystem there's a great probability of a second drive failure during the rebuild process. I have experienced such failures multiple times in the last few years. Have you considered RAID6 or 10 to lower the risk of data loss in a worst case situation.
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