Forum OpenACS Q&A: More complicated RAIDs

8: More complicated RAIDs (response to 7)
Posted by Andrew Piskorski on
Steve, RAID 5 is typically slower for writes than RAID 1. I think it is supposed to be just as fast for reads. But of course RAID 10 generally gives better performance than either RAID 1 or 5, for both reads and writes. :)

Incidentally, I've never seen any good performance comparison for more complicated RAID setups using more disks. Say you wanted one volume with the fastest IO you could get. The traditional answer is, "Buy 4 of the very fastest SCSI disks you can get, and run them in RAID 10."

But, that is sort of a silly answer, because it assumes you are using only 4 disks, but in reality those 4 15,000 rpm SCSI disks might cost as much 12 7,200 RPM IDE disks. So is there some more complicated RAID configuration that would give you faster IO using those 12 IDE disks? I'd bet there is, but I don't know.

Basically, for different types of disks (size, speed, and cost), what are the optimum RAID configurations for various different trade-off point of storage vs. speed vs. cost? If anyone's done a good study on that anywhere, I'd like to see it.

Of course if you are mounting all the disks locally, there are practical limits too how many you can stuff into one box. But once you start talking about stand-alone storage boxes talking over a HyperSCSI, iSCSI, or Fiber Channel SAN to your other servers, the possibilities start looking much more open ended...