kern/57675: persistent file system snapshots aren't obvious

[campbell+netbsd%mumble.net@localhost]

>Number:         57675
>Category:       kern
>Synopsis:       persistent file system snapshots aren't obvious
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Oct 25 16:40:00 +0000 2023
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, ...
>Organization:
The NetBSD Fssconfiguration
>Environment:
>Description:
If /path/to/backup resides on the file system mounted at /path, then

# fssconfig fssN /path /path/to/backup

creates a persistent snapshot, which continues to accrue rollback data even after you unconfigure fssN, unmount the file system and remount it, reboot, &c., until you unlink /path/

In this state, fssconfig -l makes it look like there are no active snapshots, and mounting the file system doesn't print anything about them, and as far as I know there's nothing short of dd | hexdump (not even dumpfs) that will print the list of active snapshot nodes.

It would be nice if there were some obvious way to remind the operator of active snapshots that are not only using space, but actively expanding whenever other writes happen on the file system.
>How-To-Repeat:

>Fix:
Yes, please!