Check out the man page for raidctl.
You want something like this:
# raidctl -A root raid0

Here is an excerpt from the manpage:
     -A root dev
             Make the RAID set auto-configurable, and also mark the set
as be-
             ing eligible to be the root partition.  A RAID set
configured
             this way will override the use of the boot disk as the root
de-
             vice.  All components of the set must be of type RAID in
the
             disklabel.  Note that the kernel being booted must
currently re-
             side on a non-RAID set.

Note the comment that it will override the boot disk.  This means it
will
"just work" to have / on raid0 even though the kernel loads from wd0a.

-Andrew
----- Original Message -----
From: "hugob" <hugob@xs4all.nl>
To: <port-i386@netbsd.org>
Sent: Friday, December 29, 2000 4:53 PM
Subject: raid config


> Question about raid configuration of NetBSD 1.5 on i386:
> I have a small boot partition "/dev/wd0a" with a (raid-enabled) kernel
on it.
> I have configured a "/dev/raid0d" device (created from wd0e and wd1e)
and put a root filesystem on it.
> How can I tell the system to boot from the small partition and
continue with the raid partition?.
> I tried putting a line "/dev/raid0d / ffs rw 1 1" into fstab on the
bootpartition but that results in an error:"specified device does not
match mounted device".
>
>
> Hugo Burm
> hugob@xs4all.nl
>