lvm cache support lost in zesty

[Julian Taylor]

grub fails to work when araid1 lvm volume exists that is cached in zesty.
grub version 2.02~beta3-4ubuntu1
The same configuration worked on yakkety

~$ sudo update-grub2
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.10.0-14-generic
Found initrd image: /boot/initrd.img-4.10.0-14-generic
Found linux image: /boot/vmlinuz-4.8.0-45-generic
Found initrd image: /boot/initrd.img-4.8.0-45-generic
Found memtest86+ image: /memtest86+.elf
Found memtest86+ image: /memtest86+.bin
Found Windows 7 on /dev/sdc4
Found Debian GNU/Linux wheezy/sid on /dev/mapper/lvm-debian
Found Ubuntu 15.04 (15.04) on /dev/mapper/lvm-root2
Found Ubuntu 14.04.1 LTS (14.04) on /dev/mapper/lvm-root3

$ sudo lvcreate --type raid1 -L 1G lvm -n testing
  Logical volume "testing" created.
~$ sudo lvcreate --type cache --cachemode writeback -L 10G -n ssdcachetest lvm/testing /dev/sda1
  Using default stripesize 64.00 KiB.
  WARNING: Data redundancy is lost with writeback caching of raid logical volume!
  Logical volume lvm/testing is now cached.

~$ sudo update-grub2
Generating grub configuration file ...
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
error: invalid segment.
grub-probe: error: disk `lvmid/OwVp3J-KH9N-RcXl-Fh3J-ZEiZ-egg4-JRffI3/7Dieap-dj1i-dHiz-eLfa-c7be-IiyB-GaHL26' not found.

removing the cache fixes the issue.

lvmid/OwVp3J-KH9N-RcXl-Fh3J-ZEiZ-egg4-JRffI3/7Dieap-dj1i-dHiz-eLfa-c7be-IiyB-GaHL26 is my boot lvm partition which is a regular uncached linear partition.
  --- Logical volume ---
  LV Path /dev/lvm/boot
  LV Name boot
  VG Name lvm
  LV UUID 7Dieap-dj1i-dHiz-eLfa-c7be-IiyB-GaHL26
  LV Write Access read/write
  LV Creation host, time ubuntu, 2015-09-27 23:22:37 +0200
  LV Status available
  # open 1
  LV Size 1.00 GiB
  Current LE 256
  Segments 1
  Allocation inherit
  Read ahead sectors auto
  - currently set to 256
  Block device 253:2

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Migas (migaxmoitax)]

This bug is still present in ubuntu 18.04.2

[Adam Kulagowski (fidor-fidor)]

This bug is still present in ubuntu 19.04

[Rhys (rhvs)]

Still happening in 20.04.

[Dimitri John Ledkov (xnox)]

grub2 used to support lvm2 cache partitions, in a buggy way, corrupting them on boot/grubenv read or write.

this was disabled.

the next upstream grub2 release has proper support for lvm2 cache partition, thus this may come back whenever 2.06 lands.

it will not be backported.

Please use separate /boot partition for grub without lvm2 caching, if you want to use lvm2 caching for your regular rootfs.