When booting a server that is suffering from a (probably unrelated) bug in multipath-tools, I get the following questions posed on the serial console during bootup:
The disk drive for /opt/vnx is not ready yet or not present.
Continue to wait, or Press S to skip mounting or M for manual recovery
Problem is, pressing "S" or "M" (even if followed by a newline) does absolutely nothing on the serial console. It does work on the KVM console, but that might not be available. You might very well need to send someone to a physical data centre somewhere in the middle of nowhere. (I ended up with powercycling the server, temporarily network booting it into a ramdisk, mounting the rootfs and commenting out the problematic entry from /etc/fstab, and rebooting again - which went fine, allowing me to later mount the missing filesystem manually via ssh.)
I found no way to make the boot finish or break out to a debug shell, and it happens before sshd starts, so you're basically stuck with no apparent way to recover. For this reason I feel the bug is quite severe.
For what it's worth I'm running 14.04.4 LTS, mountall package version is 2.53. The kernel command line is «BOOT_IMAGE=/@/boot/vmlinuz-3.13.0-77-generic root=UUID=2a141a43-97b0-4084-8816-ed1c41ac43e0 ro rootflags=subvol=@ console=tty1 console=ttyS0,115200n8».
Tore
Small correction to what I wrote above, what actually appears on the serial console is the following (note "keys:"):
The disk drive for /opt/vnx is not ready yet or not present.
keys:Continue to wait, or Press S to skip mounting or M for manual recovery