linux-kernel - Re: Future of md multipath

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Message-ID: <18864.46425.242906.213929@notabene.brown>
Date:	Fri, 6 Mar 2009 16:32:09 +1100
From:	Neil Brown <neilb@...e.de>
To:	martin f krafft <madduck@...ian.org>
Cc:	Christoph Hellwig <hch@....de>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Future of md multipath

On Thursday March 5, madduck@...ian.org wrote:
> also sprach Neil Brown <neilb@...e.de> [2009.03.05.0349 +0100]:
> > I'm in favour of deprecating it.  Whenever I hear of people using it I
> > suggest dm-multipath, but there could people I don't here about.
> > I think the upcoming mdadm-3.0 will need a compile-time option to
> > enable multipath, so people will only get it if they really really
> > want it.
> 
> How do you propose a distro to deal with this? Keep it enabled for
> a certain time (with a deprecation warning?) and then just to close
> the doors on users who still need it?
> 
> How long will you support it after deprecation?
> 
> I understand that dm-multipath is more favourable, but unless
> there's a riskless way to convert mdadm multipath into dm-multipath
> on old systems, I don't think we have the option of deprecating it,
> unless deprecation lasts for several years.

Always the practical one, aren't you :-)

I wonder how hard it would be to get mdadm to assemble a multipath
using the 'dm' code rather than the 'md' code....
Or to get the md layer in the kernel to hook in to the dm multipath
implementation.

I think it is probably worth putting in a printk now to say "You
should probably be using dm-multipath".  But we probably do need to
leave the code there for a while longer..

NeilBrown
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