Re: ZFS rollback snapshot general question

On Mon, Jun 22, 2026 at 12:57:23PM -0400, Bobb Shires wrote:
> Recently upgraded my 14.4 installation to 15.1 and something went very wrong, so I decided to rollback the snapshot the installer script made for me (THANK YOU).
> 
> When I hit enter on the rollback command the screen went completely blank, there was some brief disk activity, and the keyboard was unresponsive (like no caps lock response etc). 
> 
> I waited a few minutes and hit the power button, then powered back on and... it worked! Back to 14.4 
> 
> My question is, I'm thinking that's not the expected behavior and I did something wrong? How and when should the ifs rollback command be invoked, and what should I expect to see?
> I was logged in as me (regular user) and did su then zfs rollback.  Thanks!

You did a zfs rollback of the current root filesystem on a running system?
I'm not surprised it went badly. I'm not sure the kernel expects filesystems
that programs are currently running out of to blow away programs with no
way to page any required pages back in, for example.

Normally when a running program, or indeed any open file, is deleted the
inode remains valid until the program ends/the file is closed. (That's why
the system call is "unlink()" and not "delete()".) A ZFS rollback would
blow away the inode. If that case isn't explicitly supported then it will
probably blow up on you.

After a rollback of the root filesystem (and /usr), what should the kernel
do about all the running programs including /sbin/init (which has a PID
of 1)? A rollback would result in probably all running programs being
killed, but that leaves no program left to do anything including ask what
shell to use for a single user mode command line.

Really, the rollback should have been blocked and not allowed.
-- 
Kevin P. Neal                                http://www.pobox.com/~kpn/
"14. Re-reading No. 13, I realize that it's quite possible I'm losing my
mind. I'm glad that for the most part I'm not aware it's happening."
  -- from "20 things I'm thankful for": Fortune, Nov 29, 2004, page 230