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Saturday, March 12th, 2022 at 12:39 pm | 1,860 views | trackback url

Note: I work for Canonical, we make Ubuntu (among dozens of other products)

In many cases, you want to be able to upgrade your Ubuntu release between different versions over time. There are many tools that allow you to do this seamlessly and without loss of function or data. One such tool is called do-release-upgrade, found in the ubuntu-release-upgrader-core package. You can move between supported LTS releases, as well as development releases.

What’s missing, is the ability to roll back from a release, for example when an application or library you rely on, has no support for the newer version of the OS release. With ZFS root on Ubuntu, you can create a snapshot before upgrading and roll back that snapshot when things do not go to plan. With VMs and containers, taking snapshots and reverting those becomes even easier.

But let’s stick with standard tools and supported mechanisms for the moment. I just did this minutes before writing this blog post, and have done this dozens of times in the past.

I have a working, Ubuntu Focal Fossa (20.04) baremetal machine I use as a reproducer for tricky customer issues using MAAS (Canonical’s baremetal provisioning product). In some cases, I need to move between MAAS versions to execute those tests, and they have to be done on baremetal, because VMs and containers can’t model the same topology.

MAAS has matured in its version support and newer versions of MAAS no longer support older versions of Ubuntu. For example, Ubuntu Bionic Beaver (18.04) supports MAAS versions up to version 2.8, but to consume a newer version of MAAS (2.9, 3.0, 3.1), you have to upgrade to Focal (20.04). Once the machine has been upgraded and running MAAS 3.1 on 20.04, you can’t test issues reported against 2.8 or Bionic. Rolling back becomes important.

So let’s do that!

First and most-important, is to make sure you have no broken packages, half-installed .deb packages or weird/custom PPAs. You can use ppa-purge to get rid of those third-party PPAs (you can put them back later), but for now, let’s just move them out of the way:

rename.ul list orig /etc/apt/sources.list.d/*.list

Note: rename.ul comes from the util-linux package on most Linux distributions.

Now we can wipe out the cached package lists for the current Ubuntu release:

rm /var/lib/apt/lists/*

We also need to transform our sources.list file in /etc/apt/ to point to the previous Ubuntu release. We can either edit the file directly, or make a copy for each release, and refer to them individually. Since I roll forward and back very often, I keep copies of both, and use a symlink to flip between them. That looks like this:

-rw-r--r-- 1 root root 3060 Mar 12 11:12 sources.list-bionic
-rw-r--r-- 1 root root 3035 Mar 12 11:12 sources.list-focal
lrwxrwxrwx 1 root root 19 Mar 12 11:12 sources.list -> sources.list-bionic

When I want to move between releases, I just remake that symlink:

ln -sf /etc/apt/sources.list-focal /etc/apt/sources.list

Note: The extension of the original file cannot include the .list extension, or it will be parsed by apt/apt-get. They have to be a different extension, as .list is significant.

For now, I’m rolling back from Focal to Bionic, so this is the correct link.

We also need to make sure we define a preference to permit us to roll back. We do this with setting the package priority in a file that lives in the /etc/apt/preferences.d directory:

Create a file in there called rollback.pref with the following contents. You can use the same logic as I did with the sources.list symlink above.

Note: The extension of the original file cannot include the .pref extension, or it will be parsed by apt, and that’s not what you want.

Package: *
Pin: release a=Bionic
Pin-Priority: 1001

This indicates that the package priority for the packages with the series ‘Bionic’ have a higher priority than the currently installed versions of those same/similar packages.

Now we can update those package lists with apt update or apt-get update as you prefer. Once the package lists have been updated, hopefully without any errors, we can execute the following to downgrade all of them to the versions we need:

apt-get -fyu upgrade --allow-downgrades

Note, this may not be foolproof, depending on what’s running on your system and how you used those packages. You may need to make note of some conflicts and do some removal/reinstall of those conflicting package to work around some up/down dependency issues, but that should be minimal. Here’s one example:

# apt-get -fyu dist-upgrade --allow-downgrades
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
Hmm, seems like the AutoRemover destroyed something which really
shouldn't happen. Please file a bug report against apt.

The following information may help to resolve the situation:

The following packages have unmet dependencies:
 f2fs-tools : Depends: libf2fs0 (= 1.10.0-1) but it is not going to be installed
E: Internal Error, AutoRemover broke stuff

To resolve this, I can do something like the following:

# apt-get --reinstall install f2fs-tools
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libf2fs-format4 libf2fs5
Use 'apt autoremove' to remove them.
The following additional packages will be installed:
  libf2fs0
The following NEW packages will be installed:
  libf2fs0
The following packages will be DOWNGRADED:
  f2fs-tools
0 upgraded, 1 newly installed, 1 downgraded, 0 to remove and 237 not upgraded.
Need to get 127 kB of archives.
After this operation, 99.3 kB disk space will be freed.
Do you want to continue? [Y/n]

Answering ‘Yes’ will resolve that conflict for me:

Do you want to continue? [Y/n] y
Get:1 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 libf2fs0 amd64 1.10.0-1 [24.6 kB]
Get:2 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 f2fs-tools amd64 1.10.0-1 [102 kB]
Fetched 127 kB in 0s (1,111 kB/s)
Selecting previously unselected package libf2fs0:amd64.
(Reading database ... 78864 files and directories currently installed.)
Preparing to unpack .../libf2fs0_1.10.0-1_amd64.deb ...
Unpacking libf2fs0:amd64 (1.10.0-1) ...
dpkg: warning: downgrading f2fs-tools from 1.11.0-1.1ubuntu1 to 1.10.0-1
Preparing to unpack .../f2fs-tools_1.10.0-1_amd64.deb ...
Unpacking f2fs-tools (1.10.0-1) over (1.11.0-1.1ubuntu1) ...

If you want to be pedantic and ensure your system will definitely boot, you can do two more things before that final reboot.

sudo update-initramfs -uk all
sudo update-grub

And that’s it! Once the packages are cleanly downgraded, you should be able to reboot the machine back into the previous Ubuntu OS release.

Update: I just noticed that immediately after my first reboot, there were more packages that needed to be downgraded. I simply re-ran the downgrade again, and these were updated to the previous versions:

apt-get -fyu dist-upgrade --allow-downgrades

I also ran the following, to remove unneeded packages after that last, clean downgrade step:

apt-get -y autoremove

Last Modified: Saturday, March 12th, 2022 @ 13:09

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