I mounted a space nfs in my server /mnt/vol_fog/
So just to be clear the actual disk volume where the bits will be stored are on a different computer than your FOG server? If yes then that will not work. Its equivelent to in windows mounting a remote directory on your windows server as the W drive, then trying to share the W: drive out so that a 3rd computer can mount it. It doesn’t work that way.
Back to your FOG server, the nfs share needs to be a block device or NFS won’t share it. An iSCSI LUN is allowed since its a block level device. I have not personally done this, but it should work.
What is your end goal here?
What device is your fog server connected to?