Re: [PATCH 000 of 5] md: Introduction

[Heinz Mauelshagen <mauelshagen@redhat.com>]

On Fri, Jan 20, 2006 at 10:43:13AM +1100, Neil Brown wrote:
> On Thursday January 19, psusi@cfl.rr.com wrote:
> > Neil Brown wrote:
> > > 
> > > The in-kernel autodetection in md is purely legacy support as far as I
> > > am concerned.  md does volume detection in user space via 'mdadm'.
> > > 
> > > What other "things like" were you thinking of.
> > > 
> > 
> > Oh, I suppose that's true.  Well, another thing is your new mods to 
> > support on the fly reshaping, which dm could do from user space.  Then 
> > of course, there's multipath and snapshots and other lvm things which 
> > you need dm for, so why use both when one will do?  That's my take on it.
> 
> Maybe the problem here is thinking of md and dm as different things.
> Try just not thinking of them at all.  
> Think about it like this:
>   The linux kernel support lvm
>   The linux kernel support multipath
>   The linux kernel support snapshots
>   The linux kernel support raid0
>   The linux kernel support raid1
>   The linux kernel support raid5
> 
> Use the bits that you want, and not the bits that you don't.
> 
> dm and md are just two different interface styles to various bits of
> this.  Neither is clearly better than the other, partly because
> different people have different tastes.
> 
> Maybe what you really want is for all of these functions to be managed
> under the one umbrella application.  I think that is was EVMS tried to
> do. 
> 
> One big selling point that 'dm' has is 'dmraid' - a tool that allows
> you to use a lot of 'fakeraid' cards.  People would like dmraid to
> work with raid5 as well, and that is a good goal.
> However it doesn't mean that dm needs to get it's own raid5
> implementation or that md/raid5 needs to be merged with dm.

That's a valid point to make but it can ;)

> It can be achieved by giving md/raid5 the right interfaces so that
> metadata can be managed from userspace (and I am nearly there).

Yeah, and I'm nearly there to have a RAID4 and RAID5 target for dm
(which took advantage of the raid address calculation and the bio to
stripe cache copy code of md raid5).

See http://people.redhat.com/heinzm/sw/dm/dm-raid45/dm-raid45_2.6.15_200601201914.patch.bz2 (no Makefile / no Kconfig changes) for early code reference.

> Then 'dmraid' (or a similar tool) can use 'dm' interfaces for some
> raid levels and 'md' interfaces for others.

Yes, that's possible but there's recommendations to have a native target
for dm to do RAID5, so I started to implement it.

> 
> NeilBrown
> -
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-- 

Regards,
Heinz    -- The LVM Guy --

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Cluster and Storage Development                   56242 Marienrachdorf
                                                  Germany
Mauelshagen@RedHat.com                            +49 2626 141200
                                                       FAX 924446
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