
The concept of parity-based RAID (levels 3, 5 and 6) is now pretty old in technological terms, and the technology's limitations will become pretty clear in the not-too-distant future — and are probably obvious to some users already. In my opinion, RAID-6 is a reliability Band Aid for RAID-5, and going from one parity drive to two is simply delaying the inevitable.
The bottom line is this: Disk density has increased far more than performance and hard error rates haven't changed much, creating much greater RAID rebuild times and a much higher risk of data loss. In short, it's a scenario that will eventually require a solution, if not a whole new way of storing and protecting data.
[...] We all know that density is growing faster than bandwidth. A good rule of thumb is that each generation of drives will improve bandwidth by 20 percent. The problem is that density is growing far faster and has been for years. While density percentages might be slowing now from 100 percent to 50 percent or less, drive performance is pretty fixed at about 20 percent improvement per generation.
[...] The disk reliability density problem is getting worse, and fast. Some vendors are using techniques such as write logging — keeping track of write on another disk during rebuild to allow the rebuild to occur faster — to get around the growing problem. Will this solve the problem for the long term, or is this the equivalent of the RAID-5 to RAID-6 fix that just delayed the inevitable problem? Personally, I think it will turn out to be just another short-term fix. The real fix must be based on new technology such as OSD, where the disk knows what is stored on it and only has to read and write the objects being managed, not the whole device, or something like declustered RAID. In essence, the disk drive layer needs to have more knowledge of what is storage, or fixed RAID devices must be rethought. Or both.
But is it reliable enough yet? Would you really give all your trust to the remote equivalent of a RAID set-up? I wouldn't.
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