Re: auto-assembling arrays without a configuration file

["Keld Jørn Simonsen" <keld@dkuug.dk>]

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.

> If the target machine and a "/dev/md0" and the drives that are moved
> are from a "/dev/md0" on the source machine, then any auto-assembly on
> the target machine has not obvious way to know which set of "/dev/md0"
> devices to assemble.

Moving a disk from one machine to another should be a special case, 
and done by hand.

> 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.

mkinitrd does not seem to handle linux raids at all. Only hardware raids
(doen by a controller).

best regards
keld
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