A question on UniteBSD moved me to add something to my previous post
with regard to ffs snapshots and backups. It is probably worth
mentioning how fsck_ffs(8) and dump(8) can directly make use of snapshot
with the built-in `-x` option, which automatizes the process of ffsX
(un)configuration.

■ fsck_ffs -x [snap-backup] 
The -x options of fsck_ffs(8) can be used in conjunction with -n to check
data integrity on a filesystem mounted rw, by comparing the filesystem
against an internally consistent atomic view of itself. The -x option
creates a FFS snapshot using a temporary backup log at the specified
path to write potential changes to the filesystem. If the check fails,
the filesystem should be unmounted and fixed. 

~ $ fsck_ffs -x /var/snap -n /
** /dev/rfss0 (NO WRITE)
** File system is journaled; replaying journal
** Last Mounted on /altroot/
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
6472 files, 322806 used, 4760097 free (2921 frags, 594647 blocks, 0.1%
fragmentation)

■ dump -x [snap-backup]
~ $ dump -0ua -f cgd.dump -x /var/snap /etc/cgd
  DUMP: Found /dev/rdk0 on /etc/cgd in /etc/fstab
  DUMP: Date of this level 0 dump: Sat Mar 19 10:00:50 2022
  DUMP: Date of last level 0 dump: the epoch
  DUMP: Dumping a snapshot of /dev/rdk0 (/etc/cgd) to cgd.dump
  DUMP: Label: none
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 52497 tape blocks.
  DUMP: Volume 1 started at: Sat Mar 19 10:00:50 2022
  DUMP: dumping (Pass III) [directories]
  DUMP: dumping (Pass IV) [regular files]
  DUMP: 52493 tape blocks on 1 volume
  DUMP: Volume 1 completed at: Sat Mar 19 10:00:52 2022
  DUMP: Volume 1 took 0:00:02
  DUMP: Volume 1 transfer rate: 26246 KB/s
  DUMP: Date of this level 0 dump: Sat Mar 19 10:00:50 2022
  DUMP: Date this dump completed:  Sat Mar 19 10:00:52 2022
  DUMP: Average transfer rate: 26246 KB/s
  DUMP: level 0 dump on Sat Mar 19 10:00:50 2022
  DUMP: Closing cgd.dump
  DUMP: DUMP IS DONE

Looking at the stdout about, pay attention to:

DUMP: Dumping a snapshot of /dev/rdk0 (/etc/cgd) to cgd.dump

This eliminates the main problems of using dump to make a full system
backup without a snapshot mounted, which is dumping the ongoing dump
itself, and possibly screw up both the filesystem and the backup if
anything goes wrong. By using -x, I can therefore directly dump /,
because under the hood dump will be making a FFS snapshot before dumping
the filesystem and mirroring the snapshot, rather than the filesystem
itself. I hadn't mentioned this in the post, since the man page says
it's experimental. On second thought, it would have been better to
include that too. Searching the mailing lists, I found a couple of
ancient posts with people reporting using dump -x reliably to carry out
backups in the log run. But I haven't experimented myself, and I didn't
want to risk, so I preferred the 'manual' approach.