On 07/12/2015 12:25 PM, Alan McKinnon wrote: > It's because the "RAID" abilities built into most motherboards are > really shitty. Very little, if any, optimization going on, no real > intelligence, and the whole thing just looks and feels like it's no more > than 2 or more volumes shoved into one group. You would probably be > better off adding both disks to ove LVM VG and telling the system to do > simple striping or mirroring.
It can also be noticeable with a slower CPU, as all tasks are offloaded to the CPU. If you are doing I/O heavy tasks (like compiling) it can be very noticeable. Also beware Highpoint cards - quite a few of them are no better than onboard RAID - their prices reflect this. > Proper hardware RAID using a proper RAID adapter is a whole different > story. Those tend to have proper firmware and control the disks > properly. They do the optimizations you expect, they will reorder writes > and do something sane with reads, and are worth the money is you need > RAID. However, most still only do a subset of desirable RAID, usually 0, > 1 and 5. When I upgraded my mythtv backend with a 9750-8i (a real RAID card ~$800 at the time I got it) the difference in I/O performance is staggering. Mine supports RAID 6 and 10 as well. > Linux software RAID, done in-kernel, is an amazing compromise. It's > almost as efficient as good hardware RAID, and covers all bases and RAID > types. Yes, it is somewhat more complex and you do have to use > management utilities when working with it. It's similarly complex to > administrating say LVM (not the same, just same order of magnitude) and > comes with the benefit of NOT costing half a server motherboard :-) It also has the benefit of being able to recover a RAID on a separate machine. If your RAID HBA goes and you can't find a replacement, you're screwed. (Comments are from my first hand experience on my mythtv backend at home, I'd tried Highpoint, motherboard fakeraid, and mdadm before getting a real RAID HBA.) Dan
