Bootoption "showmounts" broken in Ubuntu 20.04 iso's

[J. Mueller]

When booting from Ubuntu 20.04-iso's (20.04, 20.04.1, 20.04.2), the bootoption "showmounts" is broken, no /cow folder will be created. Because of the missing /cow folder casper-snapshot is not working (--cow is mandatory).

A cat /proc/cmdline shows the passed "showmounts". With 18.04.x-iso's it works correct.

[Michael Hudson-Doyle (mwhudson)]

I guess this was broken in https://launchpad.net/ubuntu/+source/casper/1.428 although that looks like it should _always_ create /cow rather than just when showmounts in passed. It's not working though.

[Dimitri John Ledkov (xnox)]

We have many different types of 20.04 isos.... which one do you mean? server-live? desktop? a flavour?

What are you trying to achieve? Are you trying to make casper-snapshot to work? or showmounts to work? Do you need either of them?

If you are trying to make casper-snapshot to work, where are you storing the snapshots?

[J. Mueller (jmuell)]

I give you a longer description to explain the situation.

>>> We have many different types of 20.04 isos.... which one do you mean? <<<
All the desktop-isos of Ubuntu (ubuntu-20.04.1.0-desktop-amd64.iso for example, older and newer too) are affected.

>>> What are you trying to achieve? <<<
At the moment i use ubuntu-18.04.1-desktop-amd64.iso, running from USB to create a customized live-system without re-creation of the whole squashfs.

Booted from USB with "showmounts", additional installed initramfs-tools_0.130ubuntu3.6-packages (all, core, bin) and the "dirty fix" of the casper-script from https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1520575 i snapshot all my changes in cpio-format.

The snapshot file must be stored as casper-snapshot.cpio.gz directly under /cdrom in the running live -system (filesystem remounted with -rw), on the USB stick it is then directly in the root of the fs. Now you can boot the live-system with "showmounts persistent" and you have a customized system without re-creation of the whole filesystem.squashfs file. The "dirty fix" from +bug/1520575 makes the deal so the snapshot file will mount properly as the upper layer. Without the fix you will end in a busy box.

>>> Are you trying to make casper-snapshot to work? or showmounts to work? <<<
Now with 20.04 i can not snapshot changes. The /cow folder does not exist when booting with "showmounts", nowhere in the running livesystem you can identify additional / deleted / changed files or folders in a separate location like /cow/upper to snapshot it.

Yes, you can create /cow and /cow/upper manually after booting. But when you create a new file, nothing happens there. In 18.04 (and older) you can see the changes at <original location> and under /cow/upper/<original location>.

My knowledge is not sufficent enough to identify the root cause. Is it a problem with newer 5.x kernels or with newer casper-scripts?

[Michael Hudson-Doyle (mwhudson)]

Oh so this commit explains why /cow is not present in newer releases https://git.launchpad.net/casper/commit/?h=40489842eb531118abe6b164a97b09ea04607fd2

Unfortunately I guess I'm the closest thing casper has to a maintainer right now and I don't understand how the snapshot stuff works :(

[Matthew Wilker (mwilker)]

I would very much appreciate this being fixed, as I would like to be able to remove overlayfs whiteouts from within a live Ubuntu environment, as well as ease finding large files wasting RAM in the tmpfs mount. The linked commit message says "/root/cow did not exist, and because /cow is tmpfs, one cannot actually move it across." However, I'm convinced that the second portion of is not actually true. If I boot a Ubuntu live USB with break=init in the command line to get to an initrd shell, I can create the /root/cow directory manually and move the /cow mount there with no problems, after which I can exit the shell and Ubuntu boots as normal, with /cow exposed properly. Would it suffice to mkdir -p this directory in casper before doing the mount -o move?