auto-assembling arrays without a configuration file

auto-assembling arrays without a configuration file

[Keld Jørn Simonsen]

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.

best regrds
keld

Re: auto-assembling arrays without a configuration file

[Keld Jørn Simonsen]

On Sat, Mar 08, 2008 at 01:40:09AM +0100, Keld Jørn Simonsen 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.

I tried 

mdadm -A --scan  --config partitions

But it did not work, error message something like raid array not found.

Best regards
keld
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Re: auto-assembling arrays without a configuration file

[Neil Brown]

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.
This becomes particularly important if you move some drives from one
machine to another.
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.

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.

NeilBrown

Re: auto-assembling arrays without a configuration file

[Keld Jørn Simonsen]

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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Re: auto-assembling arrays without a configuration file

[Andre Noll]

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On 14:48, Keld Jørn Simonsen wrote:

> Hmm, I am still looking for a way to boot a linúx raid as root.

Here's an init script that works for me on a server that has its root
partition on a software raid5 on /dev/md1. It uses busybox for all
commands except mdadm.

	#! /bin/sh
	export PATH=/bin:/sbin
	echo initramfs startup

	cat > /etc/fstab << EOF
	proc /proc proc defaults 0 0
	sysfs /sys sysfs rw 0 0
	EOF

	cat > /etc/mdev.conf << EOF
	null 0:0 666
	ptmx 0:0 666
	EOF

	mount /proc
	mount /sys
	mdev -s
	mknod /dev/md1 b 9 1
	echo 'DEVICE partitions' > /etc/mdadm/mdadm.conf
	mdadm -E --scan >> /etc/mdadm/mdadm.conf
	mdadm -A --scan || sh
	mount -oro /dev/md1 /mnt || sh
	mount --move /proc /mnt/proc
	mount --move /sys /mnt/sys
	cd /mnt
	mount --move . /
	exec chroot . /sbin/init 2 > /dev/console 2>/dev/console < /dev/console

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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Re: auto-assembling arrays without a configuration file

[Neil Brown]

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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