Adding two 3TB disks to the 8TB array.

Add 3TB disk and grow md6

mdadm --add /dev/md6 /dev/sdh
mdadm --grow /dev/md6 --raid-devices=5

Extend LUKS

cryptsetup resize /dev/mapper/crypt-md6

Extend Logical Volumes

pvresize /dev/dm-6
lvextend -L +1T /dev/vg-md6/media
resize2fs /dev/vg-md6/media &
# lvextend -L +300G /dev/vg-md6/home
# resize2fs /dev/vg-md6/home &