Re: grub-probe: "found two disks with the index 3 for RAID md2" after changing superblock 0.9 to 1.0

[NeilBrown <neilb@suse.de>]

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On Tue, 16 Sep 2014 04:00:24 +0000 Andy Smith <andy@strugglers.net> wrote:

> Hi Neil,
> 
> On Tue, Sep 16, 2014 at 07:54:58AM +1000, NeilBrown wrote:
> > On Mon, 15 Sep 2014 17:12:52 +0000 Andy Smith <andy@strugglers.net> wrote:
> > > On Mon, Sep 15, 2014 at 05:00:37PM +1000, NeilBrown wrote:
> > > >   mdadm --zero-super --metadata=0.90 /dev/sda3
> > > > 
> > > > will remove any 0.90 metadata.
> > > 
> > > Trying that whole the array is running:
> > > 
> > > mdadm: Couldn't open /dev/sda3 for write - not zeroing
> > > 
> > > This needs to be done with the array stopped, presumably?
> > 
> > It's preferred, but if you are really confident (and I think you are), then
> > adding --force is justified and should make it work.
> 
> Yep, that worked, thank you! No more complaints from grub-probe.
> 
> Do you consider it a bug that mdadm does not zero the old metadata
> when upgrading it to a new version?

No, but it might be excessive caution.  I think it was a deliberate decision
to make it (relatively) easy to revert if anything when wrong.

mdadm always prefers the more recent metadata, so the old metadata is
effectively invisible to mdadm.  Not to grub though.

> 
> Or do you consider it a documentation issue to explain that it may
> be necessary to manually do it?

Maybe.  Though I really don't want to impose that sort of burden on users.
Things should  "just work".  Maybe I should  bite the bullet and just erase
the old metadata.... if I do it *after* the array is assembled it should be
safe.
The code might be a bit clumsy but that isn't the end of the world...

I've made a not on my to-do list :-)

Thanks,
NeilBrown


> 
> I think I would've liked grub-probe at least in verbose mode to say
> which version of metadata it was reading from where. That would've
> been immediately enlightening, so I'll see if they would accept a
> patch for that…
> 
> Cheers,
> Andy


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