Re: mdadm: Patch to restrict --size when shrinking unless forced

[Wakko Warner <wakko@animx.eu.org>]

NeilBrown wrote:
> On Mon, Oct 09 2017, Wakko Warner wrote:
> 
> > Phil Turmel wrote:
> >> A certain amount of the trouble with all of this is the english meaning
> >> of "grow" doesn't really match what mdadm allows.
> >> 
> >> Might it be reasonable to reject "--grow" operations that reduce the
> >> final array size, and introduce the complementary "--reduce" operation
> >> that rejects array size increases?
> >> 
> >> Both operations would share the current code, just apply a different
> >> sanity check before proceeding.
> >> 
> >> mdadm would then at least not violate the rule of least surprise.
> >
> > As a general user of md raid and as a reader of the list, I would agree that
> > this would be a better solution.  Thinking in terms of lvm, there's lvreduce
> > and lvextend.  IMO, --force wouldn't be needed for --reduce (I was orginally
> > thinking of --shrink)
> >
> > On a side note, is it possible for the lower layers to know what the last
> > used sector is?  IE lvm ontop of raid and has 10% allocated and the last
> > sector is around the 10% mark.  (If this were possible --force would be,
> > required if shrinking would result in inaccessible data)
> 
> No it isn't.  I've occasionally thought of adding functionality so that
> the a device could ask its client (e.g. filesystem, lvm, etc) if
> shrinking is OK - but it hasn't happened yet.

That's what I thought but wasn't sure.  Thanks.

> > I recently did a shrink of 4x 2tb drives so that I could replace the 2tb
> > drives with 80gb drives (yes, big shrink!)  Would have been nice for mdadm
> > to know the smallest size was that wouldn't destroy my lvm volumes that were
> > on top.
> 
> Guess, try, see if data is still accessible.  If not, revert the change.
> If you have a filesystem on the raid, fsck will complain if you made it
> too small.  I don't know what you would try with lvm.  pvscan?

Fortunately for me, my first try worked.

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