How to clean up full /boot safely?

[Colin Watson]

On Mon, Feb 12, 2018 at 04:08:52PM +0100, Liam Proven wrote:
> Your paranoid assumption that I am attacking your use of language
> appears to have blinded you to my actual point.

If that was your point then you rather obscured it by peppering your
reply with air-quotes of all my qualifiers.  Please don't do that again.

> Which is this stuff is _not guaranteed_. It is _not_ known safe,
> absolutely reliable, rock-solid "this will just work if your computer
> is functioning".

I'm almost never willing to tell you on a mailing list that something is
absolutely guaranteed, because for a mailing list post I'm not going to
put the research time in to back that sort of claim, and I don't want
the liability of somebody then coming back and blaming me if I made some
kind of mistake, or even if they made some kind of mistake and didn't
realise it.  (Somebody who's paying me for my time will obviously get
more effort put into my replies.)  If you don't like that, you can stop
reading anything else I write, because on the whole it's what you're
going to get.

"Any express or implied warranties, including, but not limited to, the
implied warranties of merchantability and fitness for a particular
purpose are disclaimed", and all that, as the BSD licence has it.

> > It's obviously fine to be technically-conservative.  But you made a very
> > specific claim which went above and beyond merely not wanting to use
> > more complex facilities:
> >
> >   "the kernel must be on something GRUB can read, i.e. a straightforward
> >   Linux filesystem."
> >
> > I called you on that claim, because it's incorrect; GRUB can read more
> > than just straightforward Linux filesystems, and has been able to do so
> > in one form or another for well over a decade.  If you meant "e.g."
> > rather than "i.e.", then that would be different.
> 
> I meant "that is".

Then you were simply wrong.  You may not *like* putting /boot on
something that isn't a straightforward Linux filesystem; I'm not going
to make you.  But GRUB supports many such configurations, and it is not
true to say that the kernel must be on a straightforward Linux
filesystem in order to work with GRUB.

I have no problem with you deciding that you don't want to use a
particular facility because you don't trust it; that's your prerogative.
I don't care about selling you on using GRUB.  But you specifically said
that a thing is not possible, when it absolutely is, so I made a factual
correction in order that other people might not take your post as a true
statement of GRUB's limitations.  I was posting to the mailing list
rather than replying privately to you, after all, so I didn't think you
were the sole audience.

(Normally that sort of "must" statement means "it definitely won't work
otherwise".  Perhaps you meant something like "the kernel must
[according to my personal policies for setting up systems] be on a
straightforward Linux filesystem".  That'd be quite different, although
I don't think it's the most natural reading.  If you in fact meant that,
then I hope you'll note the ambiguity and clarify.)

> A Linux kernel software RAID is _not_  a straightforward Linux
> filesystem. A ZFS volume set isn't. Anything encrypted or striped or
> mirrored isn't.

Of course not.  Nevertheless, they're supported by GRUB.

Notwithstanding my second paragraph above, I am extremely confident in
GRUB's LVM support, for instance, and I would be happy to say that it
will just work in the absence of hardware failures or configuration
errors.  (There is one caveat, which is that the small number of
operations in GRUB that require *writing* to the block device aren't
currently implemented for LVM, mainly due to an abundance of caution
about the risks of getting write support wrong; fortunately, the only
such operations are a few obvious partition-table-modification commands
for emergency use, and the "save_env" command for persisting variables
in an environment block file, and none of that is essential
functionality.)

I'm not prepared to make that sort of claim about other configurations
such as RAID or ZFS, simply because I haven't spent much time with those
parts of GRUB beyond a few passing bug-fixes and I don't personally use
them.  To my mind that says nothing about their reliability; I'm just
not personally prepared to say anything about them beyond having seen
them work in development environments.  I think it's nevertheless worth
me pointing out that they're supported upstream, though, because that
may very well simplify somebody's life.

-- 
Colin Watson                                       [cjwatson at ubuntu.com]