Kernel Panic from 3.19.0-68-generic VFS: Unable to mount root fs on unknown-block(0,0)

[Thomas A. F. Thorne]

This is the same machine as I just reported bug 1618771 against. I am raising this bug by hand to report the kernel panic that I was talking about in 1618771.

A virtual machine running on a Windows Server 2012R2 Hyper-V system rebooted as part of an automated upgrade last night. It seems to have just installed the new 3.19.0-68-generic kernel. The system failed to boot and instead kernel paniced.

As part of investigating the problem we tried booting the previously installed kernel (66 I think) and could bring up the system OK. It asked to perform a disk check on / and /boot as it came up, which seemed to go fine. Once we were running a `df -h` showed that /boot was 100% full with no space left at all. To improve the system an `apt-get autoremove` was run which uninstalled a bunch of older kernels, freeing just over 100MB of space from /boot. Power cycling the system allowed us to boot successfully under the new 3.19.0-68 kernel.

We have an image of the failed boot message & I will type out a few of the lines here:

VFS: Cannot open root device "mapper/warden--vg-root" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
0b00 1048575 sr0 driver: sr
Kernel panic - not syncing: VFS: Unable to mount root fs unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.19.0-68-generic #76~14.04.1-Ubuntu
Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090006 05/23/2012