Making gmirror metadata cooperate with gpt metadata

[Abner Gershon]

Disclaimer, I am not a programmer but have tinkered with shell and perl
scripts. Having said that, I think many would appreciate the option of
using gmirror with disks larger than 2T.

The reason this is currently impossible is due to GPT and gmirror both
trying to store metadata in the last disk sector.

MBR does not allow for partitioning of disk greater 2T.

Workaround to gmirror partitions rather than whole disk is less than ideal.

Lines 254-293 of geom_mirror.c « mirror « geom « lib - src - FreeBSD source
tree
<https://cgit.freebsd.org/src/tree/lib/geom/mirror/geom_mirror.c?h=releng/13.0>
relate
to clearing and storing metadata on last sector of disk.

What would be the ramifications of altering gmirror to store metadata on
the 2nd to last sector?

Am I looking at the correct region to alter this code?

-Abner