Add the "-t" option to geom(8) utility, to display geoms hierarchy.
Sample output:

% geom -t
Geom Class Provider
da0 DISK da0

da0            PART       da0s1
  da0s1        PART       da0s1a
    ffs.da0s1a VFS
    da0s1a     DEV
  da0s1        DEV
da0            DEV

da1 DISK da1

swap           SWAP
da1            DEV

cd0 DISK cd0

cd0            DEV

Reviewed by: oshogbo
Approved by: re (kib)
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17151