Mdadm raid1 on NSA325V2 is not reassembled on reboot

Installing and Using OpenWrt
1

I have two 4TB HDDs, and I want to create a RAID 1 with them. I did that using mdadm, but after a reboot it was gone and I had to reassemble it manually.

2

have you created a mdadm.conf ?

3

Where i need to create it?

4

it's usually located in /etc

5 
ARRAY /dev/md0 level=raid1 num-devices=2 metadata=1.2 name=OpenWrt:0 UUID=5ba5b9d8:efc48340:f2002d7b:6ea5cb4a
   devices=/dev/sda1,/dev/sdb1

this is in /etc/mdadm.conf

6

do i need to create an /etc/mdadm/mdadm.conf ?
or do i need to edit /etc/config/mdadm ?