I was going to add a second NIC and IP to a VMware virtual machine. I saw two NICs seemingly unused, no IP configured. If there is no IP assigned, they must be unused. I removed the 1st seemingly unused NIC. and the console was suddenly filled with these messages every few seconds.
Message from syslogd@SERVERNAME at Jun 1 11:19:32 ... kernel:unregister_netdevice: waiting for eth2 to become free. Usage count = 1
It looks like some process was using and locking on to the NIC. It turns out the VM was VCS clustered with another VM and the two seemingly unused NICs are used for heartbeat.
In my defence, there are 3 things I do before I decide a NIC is unused.
Is there an IP assigned?
ip a s ifconfig
Is there a configuration?
What portgroup is the NIC connected?
In this case, there was no IP assigned and there was no configuration. Both NICs were connected to the same portgroup as the first NIC.
If I had setup the VCS cluster, I would have known it. In other VCS clusters that I setup, I created portgroup, appropriately labelled for the heartbeat.
That way, if anyone was to look at which portgroup the NICs are connected to, it could give them a hint that they are used for something.