Comment # 23 on bug 1020283 from Andrei Borzenkov

(In reply to Thiago Macieira from comment #22)
> (In reply to Franck Bui from comment #20)
> > Could anybody post the result of a failing case with debug log enabled (add
> > "debug printk.devkmsg=on" on the kernel command line before booting) ?
> 
> That's impossible for me. Because "failing" means "failing to mount root
> filesystem", nothing gets saved to /var/log. I can't get the log that it did
> generate when I ran like that.
> 

If it fails to mount root, add rd.shell kernel parameter, you should be dropped
into dracut shell. In which you can mount USB stick and copy dmesg there. You
may need to regenerate initrd to include usb-storage and stick filesystem
kernel modules and to pass log_buf_len=16M (or larger) to ensure kernel printk
buffer is large enough.