On Dienstag, 2. April 2013 11:36:13 Peter Landmann wrote: > Hi, > > i'm a university student in end phase and considering to write my master > thesis about the md raid performance issues and to implement a prototype to > solve it. > > What i have done and know: > 1. I wrote a (internal) paper to measure raid performance with SSDs with > freebsd software raid implementations and md raid under linux. I tested > RAID 0 and RAID 5 with up to 6 Intel SSDs (X25-M G2, each 20k Write and 40k > read OPS) and esp for RAID 5 it doesn't scaled. With my fio and general > environment (bs 4k, iodepth 256, direct=1, randomwrite, spare capacity > 87,5%, noop scheduler, latest mainline kernel from git, amd phenom II 1055T > 2,8 GHz, 8GB ram) i got SSDs IOPS > 3 14497.7 > 4 14005 > 5 17172.3 > 6 19779 > > 2. AFAIK the main problem is that md uses only one write thread for each > raid instance and their is a patch in work but still not availible. > > So my questions: > 1. Is this problem solved (i know it isn't in mainline)? Is there still some > work to do? > 2. If not solved: Why isn't solved already (time? technical problem? > priority? Not solvable?) > 3. Is it the only problem? With my tests i captured detailed cpu stats and > no cpu core was nearly at its capacity. So there are known other big > reasons for perfomance issues? > For example: 6 SSD randomwrite: > CPU %usr %nice %sys %iowait %irq %soft %steal %guest %idle > all 1,17 0,00 12,67 12,71 3,27 3,05 0,00 0,00 67,13 > 0 1,41 0,00 7,88 15,42 0,07 0,15 0,00 0,00 75,07 > 1 0,00 0,00 38,04 3,14 19,20 18,08 0,00 0,00 21,54 > 2 1,50 0,00 7,55 14,78 0,07 0,02 0,00 0,00 76,08 > 3 1,09 0,00 7,31 12,15 0,05 0,02 0,00 0,00 79,38 > 4 1,35 0,00 7,41 12,94 0,07 0,00 0,00 0,00 78,23 > 5 1,65 0,00 7,78 17,84 0,12 0,03 0,00 0,00 72,57 > > 4. Is this (bringing the raid performance to or near the theoretically > performance) a work that a man can archieve in less then 6 months without > practical experience in kernel hacking (and i'm not a genuis :( ) I would start with testing, what's already available. Check out Shaohua Li's <shli@xxxxxxxxxx> post: raid5: create multiple threads to handle stripes with patches, that are available for linux-next: https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=1ae2eeac074fa4511715d988c3fac95b338d00c0 Next, check, if you can address Stan Hoeppner's remarks from that thread. Compare test runs with variation in # of cores and numa affinity, nothing else. Be careful about the SSD state regarding wear leveling. Provide performance comparison charts. Given the already existing work done in this area, I would say, this is easily achievable within the given time frame, since only the automatic numa affinity adjustments are left as a new task. Good luck. Cheers, Pete -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
