mdadm/raid5, spare disk or spare space

[Anton Gavriliuk <antosha20xx@xxxxxxxxx>]

Hi

Here is raid5 setup during build time

    Number   Major   Minor   RaidDevice State
       0     259        3        0      active sync   /dev/nvme3n1
       1     259        4        1      active sync   /dev/nvme4n1
       3     259        5        2      spare rebuilding   /dev/nvme5n1

       4     259        6        -      spare   /dev/nvme6n1
       5     259        7        -      spare   /dev/nvme7n1

I'm asking because during build time according to iostat or sar,
writes occurs only on /dev/nvme5n1 and never on /dev/nvme3n1 and
/dev/nvme4n1.

So if parity is distributed in mdadm/raid5, why mdadm writes only on
/dev/nvme5n1 ?