modifying a running array

[Lieven Van Acker <lieven.vanacker@ugent.be>]

Hey folks,

I didn't find any direct pointers on this issue, so maybe you experts
can advice me...

I was setting up a system, had only one disk at setup time but wanted to
configure raid 1 in advance...

so I used 

mdadm --create /dev/md0 --force --run --level=1 --raid-devices=1
/dev/sda1

to create my array.

Now as I have the second disk available, after duplicating the partition
table I wanted to add add /dev/sdb1 to the array. 

But 

mdadm /dev/md0 -a /dev/sdb1 

adds this device as a spare one, so the number of active devices stays
1. I could use the build command to reinitialise the array, but I wonder
if there's a way to extend the raiddevice while it's running?

Is there any way to arrange this?

Regards,

Lieven