Modprobe doesn't create nbd device

Question

I am running a server with Debian 8, and after running modprobe nbd, the program runs just fine. However, the device is not created since /dev/nbd0 does not exist. What should I do to get modprobe to run properly?

Answer 1

I have a partial answer that effectively works around your problem but does not conclusively explain why the problem happened in the first place.

Resolution

Run these commands:

sudo rmmod nbd 
sudo mount -t devtmpfs none /dev 
sudo modprobe nbd 
ls /dev/nbd*

The final command should look like this:

root@node51 [~]# ls /dev/nbd*
/dev/nbd0  /dev/nbd1  /dev/nbd10  /dev/nbd11  /dev/nbd12  /dev/nbd13  /dev/nbd14  /dev/nbd15  /dev/nbd2  /dev/nbd3  /dev/nbd4  /dev/nbd5  /dev/nbd6  /dev/nbd7  /dev/nbd8  /dev/nbd9

Explanation

We determined in chat that /dev was not being updated because it was mounted as tmpfs instead of as devtmpfs.

You can check the second column of the following command to see if /dev is mounted as tmpfs or devtmpfs:

df -T /dev

Without devtmpfs, only the device and character files defined during the initial boot would be populated in /dev. devtmpfs allows devices to be added and removed after boot.

It's not possible to unmount /dev with umount /dev because the special files in there are in use, but it is possible to load a fresh devtmpfs over the existing /dev mount.

sudo mount -t devtmpfs none /dev mounts a devtmpfs over the existing mount at /dev.

Now, when you do sudo modprobe nbd, the device files /dev/nbd0 through /dev/nbd15 get populated in /dev.

Cause

I did not determine why /dev was mounted as tmpfs instead of devtmpfs or why udev wasn't running. devtmpfs should have been the default.

This answer on Server Fault might have some leads.