https://unix.stackexchange.com/questions/88654/what-does-mdadms-spare-number-mean
https://wiki.bandaancha.st/RAID_y_LVM_en_Linux https://zackreed.me/software-raid-5-in-debian-with-mdadm/
https://zackreed.me/adding-an-extra-disk-to-an-mdadm-array/