`devfs` runs in fully capacity continuously Run df and it reports from MacOS:

    Filesystem    1024-blocks     Used Available Capacity iused               ifree %iused  Mounted on
    /dev/disk1s1    118284248 89296708  25200204    78% 1280499 9223372036853495308    0%   /
    devfs                 186      186         0   100%     644                   0  100%   /dev
    /dev/disk1s4    118284248  3145860  25200204    12%       3 9223372036854775804    0%   /private/var/vm
    map -hosts              0        0         0   100%       0                   0  100%   /net
    map auto_home           0        0         0   100%       0                   0  100%   /home
    /dev/disk2s1    126712832 29139200  97573632    23%  227650              762294   23%   /Volumes/Transcend


The devfs entry occupies physical space of 186, while it runs persistently in full capacity on my Mac. 

What's the mechanism behind it?
It is a shim to hold just the directory entries needed to abstract the hardware devices and software outlets that the system mounts over /dev

Pay it no mind.