update-grub chokes on ZFS snapshots, causing boot stuck on memtest86+

[Michael Smith]

Hi,
it all started when Ubuntu desktop suddenly got stuck on memtest86+ during boot, after months of reliable service. It turns out "update-grub" is extremely slow and must have been cut short. Letting it re-run its job in full eventually fixed the problem.

As I am using zfs-auto-snapshot (and ZFS-as-root), there are lots and lots of ZFS snapshots. "update-grub" tries to mount each one and times out looking for a bootable image therein. I configured apt and snap to auto-update from cron. Presumably, some kernel update triggered update-grub, which ran way longer than expected (without any user interaction) and got cut short by an unrelated reboot, therefore leaving the system without a boot entry, so that it fell back on memtest86+.

Please tell me which log files you might need. (I've got them all snapshotted.)

$ lsb_release -rd
Description: Ubuntu 19.10
Release: 19.10

$ apt-cache policy grub2-common
grub2-common:
  Installiert: 2.04-1ubuntu12.2
  Installationskandidat: 2.04-1ubuntu12.2
  Versionstabelle:
 *** 2.04-1ubuntu12.2 500
        500 http://de.archive.ubuntu.com/ubuntu eoan-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.04-1ubuntu12 500
        500 http://de.archive.ubuntu.com/ubuntu eoan/main amd64 Packages

$ apt-cache policy zfs-auto-snapshot
zfs-auto-snapshot:
  Installiert: 1.2.4-2
  Installationskandidat: 1.2.4-2
  Versionstabelle:
 *** 1.2.4-2 500
        500 http://de.archive.ubuntu.com/ubuntu eoan/multiverse amd64 Packages
        500 http://de.archive.ubuntu.com/ubuntu eoan/multiverse i386 Packages
        100 /var/lib/dpkg/status

$ apt-cache policy zfsutils-linux
zfsutils-linux:
  Installiert: 0.8.1-1ubuntu14.4
  Installationskandidat: 0.8.1-1ubuntu14.4
  Versionstabelle:
 *** 0.8.1-1ubuntu14.4 500
        500 http://de.archive.ubuntu.com/ubuntu eoan-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     0.8.1-1ubuntu14 500
        500 http://de.archive.ubuntu.com/ubuntu eoan/main amd64 Packages

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: grub2-common 2.04-1ubuntu12.2
ProcVersionSignature: Ubuntu 5.3.0-55.49-generic 5.3.18
Uname: Linux 5.3.0-55-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu8.9
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat May 30 23:32:27 2020
InstallationDate: Installed on 2019-10-31 (212 days ago)
InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)