On 2017/1/17 上午11:10, Stan Hoeppner wrote: > On 01/16/2017 08:35 PM, Jake Yao wrote: >> I have a raid5 array on 4 NVMe drives, and the performance on the >> array is only marginally better than a single drive. Unlike a similar >> raid5 array on 4 SAS SSD or HDD, the performance on array is 3x >> better than a single drive, which is expected. >> >> It looks like when the single kernel thread associated with the raid >> device running at 100%, the array performance hit its peak. This can >> happen easily for fast devices like NVMe. > The md raid personalities are limited to a single kernel write thread. > Work is in progress to alleviate this bottleneck by using multiple write > threads. When it will hit mainline I don't know. If you want 8 writing threads, and your md raid5 device is /dev/md0, you may have a try with, echo 8 > /sys/block/md0/md/group_thread_cnt > >> This can reproduced by creating a raid5 with 4 ramdisks as well, and >> comparing performance on the array and one ramdisk. Sometimes the >> performance on the array is worse than a single ramdisk. >> >> The kernel version is 4.9.0-rc3 and mdadm is release 3.4, no write >> journal is configured. >> >> Is this a known issue? It was, but you are on 4.9 kernel, group_thread_cnt should work for you. Coly -- 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
