Configuring RAID1 after install

[Ewan Mac Mahon]

On Sun, Mar 19, 2006 at 09:12:43AM +1100, Paul Dwerryhouse wrote:
> Yes, you can. Build the raid, but force it to use only the new device
> (ie, it will be running as a degraded raid).
> 
> Essentially, it will be something like this (assuming old partition is
> /dev/hda1 and new partition is /dev/hdc1)
> 
> mdadm --create /dev/md0 --level=1 --force --raid-devices=1 /dev/hdc1
>
That's slightly, but critically wrong; if you specify --raid-devices=1
it will create the array to expect to have only one drive in total
(obviously nonsense) rather than a two drive array with one missing. The
mdadm man page gives all the details, but in short you create a two
drive array, but use the word 'missing' in place of the second device,
so something like this:

mdadm --create /dev/md0 --level=1  --raid-devices=2 /dev/hdc1 missing


> It will be handy if you can ensure that you can boot off a CD before
> doing any of this; it's easy to make a mistake and render it unbootable.
>
Absolutely. that's very good advice.

Ewan
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