Re: Don't prevent removal of devices that break raid reqs

[Chris Mason <chris.mason@oracle.com>]

On Tue, Nov 15, 2011 at 11:37:13AM +0200, Ilya Dryomov wrote:
> On Thu, Nov 10, 2011 at 09:21:00PM -0500, Chris Mason wrote:
> > On Thu, Nov 10, 2011 at 05:32:48PM -0200, Alexandre Oliva wrote:
> > > Instead of preventing the removal of devices that would render existing
> > > raid10 or raid1 impossible, warn but go ahead with it; the rebalancing
> > > code is smart enough to use different block group types.
> > > 
> > > Should the refusal remain, so that we'd only proceed with a
> > > newly-introduced --force option or so?
> > 
> > Hmm, going to three devices on raid10 doesn't turn it into
> > raid1.  It turns it into a degraded raid10.
> > 
> > We'll need a --force or some kind.  There are definitely cases users
> > have wanted to do this but it is rarely a good idea ;)
> 
> I'm not sure about use cases Chris talks about, but sans those I think
> we should prevent breaking raids.  If user wants to downgrade his FS he
> can do that explicitly with restriper.  As for the relocation code
> 'smartness', we already have a confusing case where balancing silently
> upgrades single to raid0.
> 
> Chris, can you describe those cases in detail so I can integrate and
> align this whole thing with restriper before it's merged ?  (I added a
> --force option for some of the transitions, probably best not to add
> another closely related one)

There are a few valid use cases where people want to be able to "break"
a raid1.  I'd put it at the very bottom of the list of interesting
things, just because I see a long list of bug reports that start with:

my FS was broken, so I told it to remove device xxyzzz, and that didn't
work so I ran --force, and then (sad story follows).

-chris