Re: ZFS rollback snapshot general question

Hi Kevin... Yes, I knew I had done something incorrectly.
Thanks for the reminder.

On Thu, Jun 25, 2026, at 21:11, Kevin P. Neal wrote:
> 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