Solved. Re: Dualboot and ZFS

[Victor Sudakov]

Warren Block wrote:
> On Fri, 23 Mar 2018, Victor Sudakov wrote:
> >>>>>
> >>>>> sysctl kern.geom.debugflags=0x10
> >>>>
> >>>> Please reconsider doing this without it being required.  It means "allow
> >>>> destroying mounted partitions" and is usually not necessary.
> >>>
> >>> It is recommended, or even required ("must") by the zfsboot(8) man page:
> >>
> >> The vagueness here is from the "If the drive is currently in use".  It
> >> could easily be replaced with "If the user wants to destroy data from a
> >> drive that is currently being used".
> >
> > In my script, the ${DISK} is supposed to be a new disk onto which a
> > system is installed (cloned actually), so any data thereon would be
> > destroyed anyway.
> >
> >> I would argue that we should
> >> remove or rewrite that, as it is misleading.  Overwriting a drive that
> >> is "in use" is generally exactly the opposite of what people want.
> >
> > I'm afraid your concern is a bit misdirected. If you can suggest a way
> > to install zfsboot without dd, and document it in zfsboot(8) then of course
> > kern.geom.debugflags=0x10 is better avoided.
> 
> That's the thing, dd should work without the debugflags setting.  If it 
> does not, it means you are trying to overwrite something in use.

To my surprise, my script works just fine without sysctl kern.geom.debugflags=0x10.
This may mean that this requirement in man zfsboot is superfluous.
Thanks for pointing this out, I've updated my bug report
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226714

Still I'm surprised that geom permits my overwriting of a VBR with dd.

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
AS43859