Lockdown adaX numbers to allow booting ?

Hi!

We have a system with 10 SATA disks. 2 disks are for the system,
8 disks drive a data pool 'bck', configured as raidz2, for backup purposes:

bck    72.8T  38.7T  34.1T        -         -     1%    53%  1.00x  ONLINE  -

The problem is that if all 10 disks are connected, the system
looses track from where it should boot and fails to boot (serial boot log):

--------------------
/boot/config: -Dh -S115200

Consoles: internal video/keyboard  serial port
BTX loader 1.00  BTX version is 1.02
Consoles: internal video/keyboard  serial port
BIOS drive C: is disk0
BIOS drive D: is disk1
BIOS drive E: is disk2
BIOS drive F: is disk3
BIOS drive G: is disk4
BIOS drive H: is disk5
BIOS drive I: is disk6
BIOS drive J: is disk7
BIOS drive K: is disk8
BIOS drive L: is disk9
//
/
--------------------

The system disks are detected as ada4 and ada5, when all disks are
plugged in.

The solution right now is this to unplug all disks of the 'bck' pool,
reboot, and re-insert the data disks after the boot is finished.

I looked into the output to kenv(1), but did not find inspiration
on how to fix this.

Now my questions:

- Shuffeling around SATA cables seems the wrong approach to fix this.
- Can we somehow lock down the disk numbering so that the system disks
  are detected as ada0 and ada1 ?
- Would
  rootdev="disk4s1a"
  in /boot/loader.conf work or is that the wrong approach ?
- How could we configure two drivers as root devices in loader.conf ?

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