Linux mdraid resync speed on 36 drive array
Found answer with Linux mdraid kernel developers. It is a bug in linux kernel.
Song Liu has confirmed that the issue is caused by blk_plug logic and provided patch for this. More details here:
https://lore.kernel.org/linux-raid/CAPhsuW7R=8XyU5wU1-NT-Eo=Hir7gV_b7+_MB+bUFx3bEecbDw@mail.gmail.com/
https://lore.kernel.org/linux-raid/[email protected]/T/#t