Autodetection using RAID1 on externa USB2-HDs

Hello!

I am trying to use my external USB2.0 hd for raid1 in my laptop, however autodetection does not work and I do not really know - but I have at least a theory.

What i did till now:
1.) set the partition-type of both partitiuons from 83 (Linux) to fd (Linux Raid Autodetection) 2.) mdadm --create /dev/md0 --level=raid1 --reaid-devices=2 /dev/hda2 /dev/sda1

This works great on creation, I can watch with /proc/mdstat the syncing process, but if I reboot the autodetection does not work. I can see that linux-raid discovers an raid (on /dev/hda2) but something returns "-22" (have forgotten what exactly) - it seems that my usb-disc (/dev/sda1) is simply not known at raid-discovery-time. Some seconds later I can see that the kernel disvcovers my usb-drive - so I think my theory could be right?

Is there a way to tell the kernel after boot where to find raid-arrays and howto handle them. Is that maybe possible with only a /boot partition and the reast on a mirrored "/" (root) partition.

Please help, I don't know any further :(

lg Clemens Eisserer
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Yes. This is exactly the same problem that others have booting from USB disk devices (or, at least, running the root filesystem on them.)

There is a way to handle this:

Make sure your kernel boots with an initrd, or similar.

Under Debian, at least, you could pretty trivially graft in a custom script that loads the USB modules, waits until the USB devices have been discovered[1], then continues to the bit where it uses mdadm to assemble the root RAID device.

Season to taste for your own distribution, of course, but I bet that pretty much all of the standard ones have support for this in some shape or other.

This also has the advantage that you don't need to rely on the kernel to do autodetection, that you can do more complex things, and that you can put some recovery tools onto the initrd as well.

Regards,
Daniel

Footnotes:
[1] Make sure you test with it disconnected at some point, though. :)

If racial discrimination were genetic, why would so many barriers to
intermarriage have been necessary?
        -- Gloria Steinem

If you really want to do this, I'd suggest stopping the RAID1 prior to shutdown, and then starting the RAID1 from rc.local, then your RAID1 wil be recognised and resynced.

Consider what will happen when you turn on laptop without USB RAID1 partner HD plugged in?

If you are trying for easy way to backup data, I would suggest rsync.

Cheers,
Grant.