Nexx WT3020 - problem with mdadm

Hi,

I have an issue with mdadm:

root@LEDE:~# mdadm --create --verbose /dev/md0 --level=stripe --raid-devices=2  --chunk=4K /dev/sda /dev/sdb
mdadm: /dev/sda appears to be part of a raid array:
       level=raid0 devices=2 ctime=Mon Apr  9 19:52:46 2018
mdadm: partition table exists on /dev/sda but will be lost or
   meaningless after creating array
mdadm: /dev/sdb appears to be part of a raid array:
       level=raid0 devices=2 ctime=Mon Apr  9 19:52:46 2018
mdadm: partition table exists on /dev/sdb but will be lost or
       meaningless after creating array
Continue creating array? y
mdadm: unexpected failure opening /dev/md0
root@LEDE:~# ls /dev/md*
/dev/md0
root@LEDE:~# lsmod |grep md
md_mod                101776  3 raid10,raid1,raid0
root@LEDE:~# ls -l /dev/md0
brw-------    1 root     root        9,   0 Apr 10 20:39 /dev/md0
root@LEDE:~# cat /proc/mdstat 
Personalities : [raid0] [raid1] [raid10] 
unused devices: <none>

It's seem that it is the same problem that:RAID not working after Update LEDE 17.01.0 final GoFlexNet

Just in case somebody stumbles into this. The mdadm uses O_DIRECT when opening the devices in /dev. O_DIRECT requires the CONFIG_KERNEL_DIRECT_IO option to be set in OpenWRT/LEDE’s configuration. Sadly CONFIG_KERNEL_DIRECT_IO is not enabled by default, so this will require you to compile a custom kernel/image with the option enabled.

Is it a good guess?

yes it is.

Try with the snapshot firmware as there it should be enabled for all (see below in the thread you linked). Also if nothing changes the next release (OpenWrt 18) should have that enabled too.

It's enabled in master (trunk) however I would be very careful as USB is far from ideal and the very limited amount of memory will not be to your advantage.

Thanks!
@diizzy, I just need to strip 2 usb key to get a faster/bigger one. No RAID calculation involved.