boot - "ALERT! /dev/mapper/ubuntu--vg-root does not exist " - Ask Ubuntu

when boot server in ubuntu 14.04 lts desktop . encounter situation , after screen tell how bios

then

gave waiting root device. common problem:   - boot args (cat /proc/cmdline)    - check rootdelay= (did system wait long enough?)    - check root= (did system right device?)   - missing modules (cat /proc/modules; ls/dev) alert!  /dev/mapper/ubuntu--vg-root not exist. dropping shell!  busybox v1.21.1 (ubuntu 1:1.21.0-1ubuntu1) built-in shell (ash) enter 'help' list of built-in commands.  (initramfs)_ 

after typed exit , system start normally.

i have searched , found out there many similar problems mine . answers have tried cannot solve situation. i'm going describe server environment , have done detailed possible , appreciate . made me annoyed because didn't special before last shutdown .

i have installed ubuntu system lvm mechanism.

sudo parted -l  

i took screenshot when system setup several weeks ago.

lsblk 

i have 2 disks , sdb volume group , logical volumes on vg kvm . think it's not matter , mention.

i've followed hints inspect :

- boot args (cat /proc/cmdline)  - check rootdelay= (did system wait long enough?)   - check root= (did system right device?)   

what i've got

cat /proc/cmdline   boot_image=/vmlinuz-4.4.0-45-generic.efi.signed root=/dev/mapper/ubuntu—vg-root ro rootdelay=30 quiet splash  

as can see , i've edited /etc/default/grub add rootdelay=30, fixed nothing.

- missing modules (cat /proc/modules; ls/dev) 

not knowing should notice hint . when list files in /dev/mapper , /dev/mapper/ubuntu--vg-swap_1 , /dev/mapper/ubuntu--vg-root there .

other information asked provide similar problem :

cat /etc/fstab 

blkid 

i have entered grub menu boot recovery mode , there 3 pairs :

ubuntu, linux 4.4.0-45-generic ubuntu, linux 4.4.0-45-generic(recovery mode) ubuntu, linux 4.4.0-42-generic ubuntu, linux 4.4.0-42-generic(recovery mode) ubuntu, linux 4.4.0-38-generic ubuntu, linux 4.4.0-38-generic(recovery mode) 

but when select version in recovery mode boot , fail busybox again . mentioned , after type exit start system , don't think should every time , , apparently server have wrong .

i want know behavior or situation cause problem ,then prevent .

liveusb boot environment :

sudo parted -l

pvdisplay

vgdisplay

lvdisplay

lvmdiskscan

it seems disks fine , looks somehow have ended initramfs has been made without lvm support.

try adding new kernel , check setup process closely errors