On 12/07/2015 22:46, Daniel Frey wrote: > 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.
Over the years I've seen similar results from Adaptect cards too (usually the models Dell ships) > >> 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. That's the downside of hardware raid. Oftentimes, if the disks found when the card boots are not it's own disks, or don't have the correct magic number in the firmware, you can be royally 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.) -- Alan McKinnon [email protected]
