Blank Screen on Reboot

If the Photon OS kernel enters a state of panic during a reboot and all you see is a blank screen, note the name of the virtual machine running Photon OS and then power off the VM.

In the host, open the vmware.log file for the VM. When a kernel panics, the guest VM prints the entire kernel log in vmware.log in the host directory containing the VM. This log file contains the output of the dmesg command from the guest, and you can analyze it to help identify the cause of the boot problem.

Example

After searching for Guest: in the following abridged vmware.log, this line appears, identifying the root cause of the reboot problem:

```
2016-08-30T16:02:43.220-07:00| vcpu-0| I125: Guest: 
    <0>[1.125804] Kernel panic - not syncing: 
    VFS: Unable to mount root fs on unknown-block(0,0)
```

Further inspection finds the following lines:

2016-08-30T16:02:43.217-07:00| vcpu-0| I125: Guest: 
<4>[    1.125782] VFS: Cannot open root device "sdc1" or unknown-block(0,0): error -6
2016-08-30T16:02:43.217-07:00| vcpu-0| I125: Guest: 
<4>[    1.125783] Please append a correct "root=" boot option; 
here are the available partitions: 
2016-08-30T16:02:43.217-07:00| vcpu-0| I125: Guest: 
<4>[    1.125785] 0100            4096 ram0  (driver?)
...
0800         8388608 sda  driver: sd
2016-08-30T16:02:43.220-07:00| vcpu-0| I125: Guest: 
<4>[    1.125802]   0801         8384512 sda1 611e2d9a-a3da-4ac7-9eb9-8d09cb151a93
2016-08-30T16:02:43.220-07:00| vcpu-0| I125: Guest: 
<4>[    1.125803]   0802            3055 sda2 8159e59c-b382-40b9-9070-3c5586f3c7d6

In this unlikely case, the GRUB configuration points to a root device named sdc1 instead of the correct root device, sda1. You can resolve the problem by restoring the GRUB GNU edit screen and the GRUB configuration file (/boot/grub/grub.cfg) to their original configurations.