There are multiple steps:
http://www.ducea.com/2009/03/08/mdadm-cheat-sheet/
https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html
https://h3x.no/2011/07/09/tuning-ubuntu-mdadm-raid56
https://developer.ibm.com/tutorials/l-4kb-sector-disks/
https://www.linuxquestions.org/questions/linux-general-1/recovering-mdadm-superblocks-713234/