On Tue, Apr 02, 2013 at 11:36:13AM +0000, 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 :( ) Hi Peter, I would not be so sure the issue (if any) is in the multi threading. Chunk size, stripe cache size, all make a difference in terms of write performances. I had a 4 HDDs (rotating) RAID-5, changing the stripe cache from the default (256, I guess) to the max possible (32768) made the writes scaling up linearly to the max theoretical (4 HDDs) from the initial one (1 HDD). Furthermore, the motherboard chipset controlling the 6 SSDs can be a bottleneck too. I've seen huge differences between different types. My suggestion would be to start with a baseline using RAID-0 (same number of SSDs or one less than RAID-5) and then see what's the maximum possible. Hope this helps, bye, pg > > Thanks in advance for your responses, > Peter Landmann > > -- > 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 -- piergiorgio -- 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
