Re: auto-assembling arrays without a configuration file

[Neil Brown <neilb@suse.de>]

On Monday March 10, keld@dkuug.dk wrote:
> On Mon, Mar 10, 2008 at 10:01:50AM +1100, Neil Brown wrote:
> > On Saturday March 8, keld@dkuug.dk wrote:
> > > I want to assemble the root partition automatically, without having 
> > > a configuration file. Is that possible? 
> > > 
> > > mdadm -A --scan 
> > > 
> > > does seem to require a configuration file.
> > > 
> > > On the other hand, I think all info needed is available in the super
> > > blocks, and a traversal of the partitions present on the system (ala
> > > fdisk -l) could give consistent naming - there seems to be no /dev/md
> > > association available in the superblock.
> > 
> > The information that is not present in the super blocks is which
> > array you want to assemble.
> 
> Yse, that is evident.
> 
> > This becomes particularly important if you move some drives from one
> > machine to another.
> 
> Moving a disk from one machine to another is not the common thing with
> raids. This is only done in special cases, and not prat of ordinary 
> operations.

I absolutely agree.  However the time when you do it you are quite
possibly trying to get something that was broken working again.  And
so you don't want any surprises.

So I encourage configurations where moving devices around will not
cause unpleasant surprises.  Tagging all arrays with the host name
helps remove these surprises.

> 
> > For that reason mdadm knows about a "homehost".  You can tag each
> > array with a hint about what host it expects to be assembled on.
> > If you run
> > 
> >   mdadm -As --homehost=`hostname`
> > 
> > then it will auto-assemble any arrays for the current host.
> > If you arrays haven't been tagged for at particular host, then
> > 
> >   mdadm -As --homehost=`hostname` --auto-update-homehost
> > 
> > will automatically tag everything that is found for the current host.
> > This is not something that should be done automatically, but it OK to
> > do one when you know you haven't done anything interesting with
> > devices.
> 
> Hmm, I am still looking for a way to boot a linúx raid as root.

Look at README.initramfs in the mdadm release.
It might not answer all your questions, but I describes in general how
it should work, and provides a simple sample you could build on.

NeilBrown
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