See https://wiki.freebsd.org/RootOnZFS/ZFSBootPartition for more
details.

Script started on Tue Jan 16 08:40:32 2018
# PS1='[script]# '

!!! These are the two MBR slices created by Windows 10, half of ada0 is available for FreeBSD !!!

[script]# gpart show ada0
=>       63  134217665  ada0  MBR  (64G)
         63       1985        - free -  (993K)
       2048    1024000     1  ntfs  [active]  (500M)
    1026048   66082816     2  ntfs  (32G)
   67108864   67108864        - free -  (32G)

!!! Now, add our FreeBSD slice !!!

[script]# gpart add -t freebsd ada0
ada0s3 added

!!! Here it is !!!

[script]# gpart show ada0
=>       63  134217665  ada0  MBR  (64G)
         63       1985        - free -  (993K)
       2048    1024000     1  ntfs  [active]  (500M)
    1026048   66082816     2  ntfs  (32G)
   67108864   67108864     3  freebsd  (32G)

!!! Create a BSD disklabel within the FreeBSD slice !!!

[script]# gpart create -s BSD ada0s3
ada0s3 created
[script]# gpart show ada0
=>       63  134217665  ada0  MBR  (64G)
         63       1985        - free -  (993K)
       2048    1024000     1  ntfs  [active]  (500M)
    1026048   66082816     2  ntfs  (32G)
   67108864   67108864     3  freebsd  (32G)

!!! Yes, the disklabel is in place !!!

[script]# gpart show ada0s3
=>       0  67108864  ada0s3  BSD  (32G)
         0  67108864          - free -  (32G)

!!! Add our ZFS partition !!!

[script]# gpart add -s 28G -t freebsd-zfs ada0s3
ada0s3a added

!!! Add our swap partition !!!

[script]# gpart add -t swap ada0s3
ada0s3b added

!!! Show the resulting disklabel !!!

[script]# gpart show ada0s3
=>       0  67108864  ada0s3  BSD  (32G)
         0  58720256       1  freebsd-zfs  (28G)
  58720256   8388608       2  freebsd-swap  (4.0G)

!!! Load the ZFS module, and optionally set the ashift property to 12 !!!

[script]# kldload zfs
[script]# sysctl vfs.zfs.min_auto_ashift=12
vfs.zfs.min_auto_ashift: 9 -> 12

!!! Create our zpool, YMMV !!!

[script]# zpool create -O mountpoint=legacy zroot /dev/ada0s3a

!!! Create our initial BE, YMMV !!!

[script]# uname -a
FreeBSD  11.1-STABLE FreeBSD 11.1-STABLE #0 r327786: Wed Jan 10 22:26:38 UTC 2018     root@releng2.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64
[script]# zfs create zroot/ROOT
[script]# zfs create zroot/ROOT/20180110-r327786
[script]# zfs set mountpoint=/tmp/zroot zroot/ROOT/20180110-r327786

!!! The BE is temporary mounted at /tmp/zroot !!!

[script]# df -ah
Filesystem                           Size    Used   Avail Capacity  Mounted on
/dev/iso9660/11_1_STABLE_AMD64_CD    701M    701M      0B   100%    /
devfs                                1.0K    1.0K      0B   100%    /dev
tmpfs                                 32M    124K     32M     0%    /var
tmpfs                                 20M     68K     20M     0%    /tmp
zroot/ROOT/20180110-r327786           27G     88K     27G     0%    /tmp/zroot

!!! Extract a minimum installation !!!

[script]# cd /tmp/zroot
[script]# tar xf /usr/freebsd-dist/base.txz
[script]# tar xf /usr/freebsd-dist/kernel.txz
[script]# tar xf /usr/freebsd-dist/lib32.txz

!!! Ensure the ZFS module will be loaded by the loader !!!

[script]# echo 'zfs_load="YES"' >> boot/loader.conf

!!! Make a note of our swap partition !!!

[script]# echo '/dev/ada0s3b none swap sw 0 0' >> etc/fstab

!! Prepare our pool for export !!!

[script]# cd /tmp
[script]# zfs unmount -a
[script]# zfs inherit mountpoint zroot/ROOT/20180110-r327786
[script]# zpool set bootfs=zroot/ROOT/20180110-r327786 zroot
[script]# df -ah
Filesystem                           Size    Used   Avail Capacity  Mounted on
/dev/iso9660/11_1_STABLE_AMD64_CD    701M    701M      0B   100%    /
devfs                                1.0K    1.0K      0B   100%    /dev
tmpfs                                 32M    128K     32M     0%    /var
tmpfs                                 20M     68K     20M     0%    /tmp
[script]# zpool export zroot

!!! Write the boot manager to the boot block in the MBR !!!

[script]# gpart bootcode -b /boot/boot0 /dev/ada0
bootcode written to ada0

!!! Write /boot/zfsboot as two separate pieces to /dev/ada0s3{,a} !!!

[script]# dd if=/boot/zfsboot of=/tmp/zfsboot1 count=1
1+0 records in
1+0 records out
512 bytes transferred in 0.000383 secs (1337758 bytes/sec)
[script]# gpart bootcode -b /tmp/zfsboot1 /dev/ada0s3
bootcode written to ada0s3
[script]# dd if=/boot/zfsboot of=/dev/ada0s3a skip=1 seek=1024
128+0 records in
128+0 records out
65536 bytes transferred in 0.043591 secs (1503439 bytes/sec)

!!! Import our pool !!!

[script]# zpool import zroot
[script]# zfs list -rt all
NAME                          USED  AVAIL  REFER  MOUNTPOINT
zroot                         742M  26.2G    88K  legacy
zroot/ROOT                    742M  26.2G    88K  legacy
zroot/ROOT/20180110-r327786   742M  26.2G   742M  legacy

!!! Everything checks out, it's time for a reboot to assess the damage !!!

[script]# exit

Script done on Tue Jan 16 08:53:07 2018