On January 16, 2013, Stan Hoeppner wrote: [snip] > This is the reason why RAID6 performs so horribly with mixed read/write > workloads. Using Thomas' example, while he was doing a streaming read > of a media file and simultaneously doing non-aligned writes from a P2P > or other application, md is performing a RMW operation during each > write, adding substantially to the seek burden on the drives. RAID5/6 > use rotating parity, so he also has an extra seek on each of two drives > occurring, competing with the read seeks of his streaming app. Consumer > 7.2K drives aren't designed to handle this type of random seek load with > good performance. Re-reading through this thread (I have a bit of spare time this weekend), I finally understood what you wrote there. I'm not normally quite this dense, and I appologise. The RMW ops and double seek penalties are really quite harsh. > If using RAID10 or RAID0 over RAID1, there is no RMW penalty for partial > stripe width writes, and no extra seek burden for the parity writes, as > described above for RAID5/6. Thus it doesn't cause the playback stutter > as the disks can service the read and write requests without running out > of head seek bandwidth as parity arrays do due to RMW and parity block > writes. > > In summary, with Thomas' old disk system, he would have most likely > avoided the playback stutter simply by using a non-parity RAID level. > > I'm constantly amazed by the fact that so many people here using parity > RAID don't understand the performance impact of these basic parity RAID > IO behaviors, and how striping actually works, and the fact that most > often they're not writing full stripes, and thus not benefiting from > their spindle count. I actually do/did have a decent understanding of how raid5 works, and why its slower than a lay-person would intuit. The extra seeking, the RMW, and for software raid, the parity calculations. What's happened is I didn't expand that up to how raid6 obviously works, with an extra parity set interleaved with the data. *sigh* Complete face palm moment. In the future, if I need more performance than what I'll get out of this setup, I'll move to smaller drives (with ERC, and low URE rates), in raid10. If I had a couple/few extra bays in this box, I'd be very tempted to go raid10 right now. As I haven't noticed any stuttering with my "old" 5.5TB (7x1TB) array, I somehwat doubt I'll notice any on my new 11TB array (7x2TB, switched to raid5, as with the backup array, I probably don't need/want the extra parity, if two drives do die at the same time, I still have a backup copy of most/all of the data, and can just restore it). Thank you Stan, Chris, Phil, and Tommy for the help and insight. It was all very helpful. -- Thomas Fjellstrom thomas@xxxxxxxxxxxxx -- 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
