What to do if your VM won't boot

After I made a bunch of changes to OpenVSwitch (see last post), I rebooted my VM and it just hung with a "random init crng" message. The last thing it did was bring up a network interface ens3 before this message appeared. I assumed it was trying to start networking, and was waiting for a timeout.

I waited for what seemed like an eternity...5 minutes? 10? Longer? And a prompt came up. I saw little to nothing in the logs, so I became concerned. How would I debug this if I have to wait that long every time I reboot?

I found a few nifty tricks to get you out of a jam if you're virtual machine won't boot.

First, if the VM will come up at all, you can enable "system-debug" with systemctl:
# systemctl enable system-debug

This service allows you to connect on tty9 to the VM as it is booting. On a virtual machine, you typically need to "send" a command to get to this terminal. So on KVM, there is a "Send Command" option and you can send keystroke commands to the VM (i.e. Ctl-Alt-Backspace, Ctl-Alt-F1...F9, et al).  This proved to be quite handy, and a sense of relief that I could actually get inside of the VM - especially if you have not snapshotted it or backed it up.

If you need to do more, or go further, you may need to interrupt the boot process and edit the line in the grub config file.  For Red Hat users, here is a link to a page that gives some direction on how to get into debug mode for a virtual machine.

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/sec-terminal_menu_editing_during_boot