raid5 - adding journal to an existing device?

mdadm manual specifies the following for create, build and grow modes:

       --write-journal
              Specify journal device for the RAID-4/5/6 array.

However:

# mdadm --grow /dev/md2 --write-journal /dev/sdc1
mdadm: :option --write-journal not valid in grow mode


Is it not yet supported, will never be supported, or am I simply trying to use it in a  wrong way?



Tried the following versions:

# ./mdadm -V
mdadm - v3.4-115-ge22fe3a - 29th November 2016

# mdadm -V
mdadm - v3.4 - 28th January 2016


Tomasz Chmielewski
https://lxadm.com