UFS alternate super block error

[Chuck Tuffli]

Recently, I enlarged the file used to back a FreeBSD 12.1-RELEASE-p7
virtual machine and thought I did all of the normal steps to grow the
file system. Something must not have gone quite right as the boot
stops with:
...
Trying to mount root from ufs:/dev/gpt/rootfs [rw]...
Setting hostuuid: 5defeee9-d2e2-46e2-b2bb-203dd6e46f1f.
Setting hostid: 0x4a7a75f3.
Starting file system checks:
Warning! Some of the devices might not be available; retrying
Waiting 30s for the root mount holders: usbus3 usbus2 usbus1
usbus0uhub1: 2 ports with 2 removable, self powered
uhub2: 2 ports with 2 removable, self powered
uhub3: 2 ports with 2 removable, self powered
..uhub0: 6 ports with 6 removable, self powered
Restarting file system checks:
Unknown error 3; help!
ERROR: ABORTING BOOT (sending SIGTERM to parent)!
2020-08-22T13:32:36.786266+00:00  init 1 - - /bin/sh on /etc/rc
terminated abnormally, going to single user mode
Enter full pathname of shell or RETURN for /bin/sh:

entering the shell and then immediately exiting allows the boot to
continue successfully. Running fsck from shell indicates there is an
error with the alternate super block:

# fsck_ffs -d /dev/gpt/rootfs
** /dev/gpt/rootfs
SUPER BLOCK VS ALTERNATE MISMATCH fs_sblkno: 24 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_cblkno: 32 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_iblkno: 40 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_dblkno: 5056 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_ncg: 162 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_bsize: 32768 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fsize: 4096 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_frag: 8 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_bmask: 0xffffffffffff8000 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fmask: 0xfffffffffffff000 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_bshift: 15 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fshift: 12 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fragshift: 3 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fsbtodb: 3 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_sbsize: 4096 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_nindir: 4096 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_inopb: 128 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_cssize: 4096 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_ipg: 80256 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_fpg: 160280 vs 0
SUPER BLOCK VS ALTERNATE MISMATCH fs_magic: 0x19540119 vs 0
VALUES IN SUPER BLOCK LSB=128 DISAGREE WITH THOSE IN
LAST ALTERNATE LSB=206440832

IGNORE ALTERNATE SUPER BLOCK? [yn]

I've tried running fsck a few times, but this issue persists. Is there
any way to fix this?

--chuck