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.
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?
Please cc me on the email as I am not on the mail list.
Thanks!
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