[linux-lvm] Best way to run LVM over multiple SW RAIDs?

[Anatoly Pugachev]

On Tue, Oct 29, 2019 at 12:14 PM Daniel Janzon
<daniel.janzon at edgeware.tv> wrote:
>
> Hello,
>
> I have a server with very high load using four NVMe SSDs and therefore no HW RAID. Instead I used SW RAID with the mdadm tool. Using one RAID5 volume does not work well since the driver can only utilize one CPU core which spikes at 100% and harms performance. Therefore I created 8 partitions on each disk, and 8 RAID5s across the four disks.
>
> Now I want to bring them together with LVM. If I do not use a striped volume I get high performance (in expected magnitude according to disk specs). But when I use a striped volume, performance drops to a magnitude below. The reason I am looking for a striped setup is to ensure that data is spread well over the drive to guarantee a good worst-case performance. With linear allocation rather than striped, if load is directed to files on the first PV (a SW RAID) the system is again exposed to the 1-core limitation.
>
> I tried "--stripes 8 --stripesize 512", and would appreciate any ideas of other things to try. I guess the performance hit can be in the file system as well. I tried XFS and EXT4 with default settings.

Daniel,

a bit more about your system? Like kernel version, io scheduler, etc..
Have you tried with recent kernels MQ (multi-queue) schedulers (noop,
deadline) ?