/usr under LVM has unexpected df/findmnt output

[Joe Prostko]

Hello,

I have been utilizing the autoinstall functionality of Ubuntu Server, and notice that when I utilize a custom LVM setup to provide different mount points that for whatever reason, /usr is not displaying concisely in df and findmnt output like it does for the other mount points. I have tried both ext4 and xfs as the filesystem, and it happens with both. I also found a forum post about this problem with other people experiencing this, and it seems to be specific to /usr. While it is mostly a cosmetic problem, I'd prefer to have things be consistent. I have tested with the live server ISO for Ubuntu 20.04, 20.10, as well as the nightly from 20201217 for hirsute hippo, all with the same result. I will provide some light information below, but will attach a tarball including a typescript with some relevant commands (like fstab output explicitly mentioning curtin), as well as relevant curtin files. I should be able to provide any other files from the build as requested.

Here is example output to show what I am talking about:

root@abc123:~# df -Th -x tmpfs
Filesystem Type Size Used Avail Use% Mounted on
/dev/mapper/vg01-root xfs 2.0G 52M 2.0G 3% /
/dev/disk/by-id/dm-uuid-LVM-Uh7xPJmOyUynq6TsmsB6AFfezHrpjyKlb3jmaS35PKR0pBpa3PmPojAN2qduw7vN xfs 6.0G 1.9G 4.2G 31% /usr
/dev/mapper/vg01-home xfs 10G 104M 9.9G 2% /home
/dev/mapper/vg01-opt xfs 10G 104M 9.9G 2% /opt
/dev/mapper/vg01-var xfs 3.0G 438M 2.6G 15% /var
/dev/mapper/vg01-tmp xfs 2.0G 47M 2.0G 3% /tmp
/dev/vda1 xfs 726M 147M 580M 21% /boot

Relevant forum post showing the problem arising with Ubuntu 20.04:

https://ubuntuforums.org/showthread.php?t=2442012

Thank you.

[Joe Prostko (joe-prostko)]

Digging into this a bit myself, it seems like this is due to a change in initramfs-tools. From my understanding of reading associated tickets and the NEWS item below, it was suggested to not use a separate /usr under LVM, or if you do, you must specify it by kernel name descriptor instead of by UUID or label.

https://sources.debian.org/src/initramfs-tools/0.139/debian/NEWS/#L43

While I never noticed the problem until Ubuntu 20.04, given the time of the change, it must have been something that was a part of previous Ubuntu versions as well for some time.

In any case, by changing /etc/fstab to reference /dev/mapper/vg01-usr in the Filesystem column, df and findmnt give the desired output. Both utilize /proc/self/mountinfo according to strace output, so naturally they all now have the desired information present. If instead /dev/vg01/usr is used in /etc/fstab, then that is shown in the df/findmnt output instead for /usr.

I think we can close this ticket, as this is outside of the scope of curtin itself.

Thanks.