boot from second disk (gmirror device) ... problems

[Fernan Aguero]

Hi

I'm using geom mirror on several FreeBSD boxes. It's been some time since
I've set these up, but I remember being bitten by this problem several times:
the problem? booting from the second disk after setting it as the first member
of a geom mirror pair.

I've always followed more or less Ralf's guide
http://people.freebsd.org/~rse/mirror/
(I say more or less because I usually mirror the second slice of my disks)

But I always end up getting trouble booting from the second disk.
(need to have to have physical access to the box).

In the previous incantation of Ralf's guide, he suggested booting from the
second disk like this:
echo "1:ad(1,a)/boot/loader" >/boot.config

Now, he replaced this in the new version of the guide by this:
echo $d2 | sed -e 's;^\([^0-9]*\)\([0-9][0-9]*\)$;1:\1(\2,a)/boot/loader;' \
    >/boot.config

where $d2 is the second disk (ad2 in my case), and this is the resulting
/boot.config:
1:ad(2,a)/boot/loader

After setting up /etc/fstab and /boot/loader.conf in my
/dev/mirror/gm0a partition
rebooting brings me to the boot loader prompt because:
1:ad(2,a)/boot/loader
is not a valid label or /boot/loader does not exist

I've tried several combinations, blindly trying to spot the correct one:
1:ad(1,a)/boot/loader
1:ad(0,a)/boot/loader

To no avail.

However, 0:ad(0,a) or 0:ad(1,a) allowed me to boot from my ad0 (first)
disk (i.e. not the gmirror device)

What's the right incantation to boot from the second disk?

I've already read boot(8) many times, and can't seem to figure
out why
1:ad(1,a)
doesn't work

bios_drive: 1 (second drive)
interface: ad
unit: 1 (second drive)
slice: a

Thanks in advance for any tip or suggestion,

Fernan

PS: my disks are ad0 (at ata0-master) and ad2 (at ata1-master).
ad2s1 (single slice) is the first member of the gm0 gmirror
gm0a is the partition containing the bootable (active) partition


--
fernan