After the update from Kubuntu 17.04 to 18.04 and a reboot, my system refused to boot and only gave me « emergency mode ». Long story short: for an unknown reason I had an empty volume « pvmove0 » that the new version did not like, and after removing it it worked.

What was in the journal?

I had a look at the system journal with this command (the LANG is to have all messages in English, which is easier to find on the net than French):

LANG="en_GB.UTF-8" journalctl -xb

I had lines like that:

May 04 20:46:05 mymachine lvm[415]: LV pvmove0: has no segment.
May 04 20:46:05 mymachine lvm[415]: Couldn't read all logical volumes for volume group fablvm.

Also, most of the LVM commands could not read the Physical Volume (PV) and the Logical Volumes (LV) on this disk: they told me that the physical volumes did not belong to a Volume Group and they had no size.

The mysterious Logical Volume pvmove0

As Kubuntu 17.10 (on an USB stick) could still read all my PV and LV, I had the idea to backup the Volume Group metadata in Kubuntu 17.10 and restore it from Kubuntu 18.04.

vgcfgbackup -f path/to/my/backup.vg my_vg

Then I looked at the backup file and (holy cow!) I saw that I had an empty Logical Volume called « pvmove0« .

pvmove0 {
 id = "Jp8y0E-RYIU-g4s0-PdUs-LEdo-N110-MLBwmQ"
 status = ["READ", "WRITE", "PVMOVE", "LOCKED"]
 flags = []
 allocation_policy = "contiguous"
 segment_count = 0
}

I searched on the net « pvmove0 » and found that someone had the same problem and remove it by:

• doing a backup of the metadata (see above)
• editing the file by removing the pvmove0 section
• restoring the edited backup with vgcfgrestore.
• rebooting (and celebrating)

Apparently, having such an empty logical volume breaks things at several levels (among others, the graphical tool system-config-lvm would not start).