Jakob Oestergaard wrote: >> >>a) write performance will be worse than RAID-5, but I believe it can >> be kept to within a factor of 1.5-2.0 on machines with suitable >> SIMD instruction sets (e.g. MMX or SSE-2); > > Please note that raw CPU power is usually *not* a limiting (or even > significantly contributing) factor, on modern systems. > > Limitations are disk reads/writes/seeks, bus bandwidth, etc. > > You will probably cause more bus activity with RAID-6, and that might > degrade performance. But I don't think you need to worry about > MMX/SSE/... If you can do as well as the current RAID-5 code, then you > will be in the clear until people have 1GB/sec disk transfer-rates on > 500MHz PIII systems ;) > RAID-6 will, obviously, never do as well as RAID-5 -- you are doing more work (both computational and data-pushing.) The RAID-6 syndrome computation is actually extrememly expensive if you can't do it in parallel. Fortunately there is a way to do it in parallel using MMX or SSE-2, although it seems to exist by pure dumb luck -- certainly not by design. I've tried to figure out how to generalize to using regular 32-bit or 64-bit integer registers, but it doesn't seem to work there. Again, my initial analysis seems to indicate performance within about a factor of 2. >>b) read performance in normal and single failure degraded mode will be >> comparable to RAID-5; > > Which again is like a RAID-0 with some extra seeks... Eg. not too bad > with huge chunk sizes. > > You might want to consider using huge chunk-sizes when reading, but > making sure that writes can be made on "sub-chunks" - so that one could > run a RAID-6 with a 128k chunk size, yet have writes performed on 4k > chunks. This is important for performance on both read and write, but > it is an optimization the current RAID-5 code lacks. That's an issue for the common framework, I'll leave that to Neil. It's functionally equivalent between RAID-5 and -6. >>c) read performance in dual failure degraded mode will be quite bad. >> >>I'm curious how much interest there would be in this, since I >>certainly have enough projects without it, and I'm probably going to >>need some of Neil's time to integrate it into the md driver and the >>tools. > > I've seen quite some people ask for it. You might find a friend in "Roy > Sigurd Karlsbach" - he for one has been asking (loudly) for it ;) :) Enough people have responded that I think I have a project... -hpa - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
