Many of the features that make Atomic Host great are due to the immutability of /usr. Things like atomic upgrades and rollbacks and efficient storage of files in an object store all require immutability. However, this immutability is also what can irritate people the most when they first start using Atomic Host. There is no yum or dnf to install programs. We are at the mercy of the content provider.

The answer of course is to use containers. Keep a pet SPC container around that will allow you to do all the configuration and hacking you need, all in the comfort of your favourite editor.

But if you’re hacking on the Atomic Host itself, containers can only take you so far. What if you want to change the version of docker on the host? Or install a new package? Or modify an installed Python script for debugging?

The latest releases of OSTree and rpm-ostree have acquired some new features that makes hacking and extending Atomic Host much easier. These features are OSTree system unlocking and rpm-ostree package layering. They both answer this need for easier Atomic Host mutation, but in a completely different way.

OSTree system unlocking

This feature was actually introduced in v2016.4 on March 23, 2016. It is already in both Fedora and CentOS Atomic Host. In a nutshell, system unlocking allows you to make temporary changes to the /usr mount. To do this, you simply use the ostree admin unlock command:

-bash-4.2# ostree admin unlock
Development mode enabled.  A writable overlayfs is now mounted on /usr.
All changes there will be discarded on reboot.

We can now do things like:

-bash-4.2# touch /usr/woohoo && echo "success"
-bash-4.2# rpm --install strace-4.8-11.el7.x86_64.rpm
-bash-4.2# strace -V
strace -- version 4.8

The ostree admin status will also have an updated status output:

-bash-4.2# ostree admin status
* centos-atomic-host 675583e7db81b59a08e0bbfb3735bddde9d768f9f4500b845799c7e97ab71091.1
    Version: 2016.0.169
    Unlocked: development
    origin refspec: centos-atomic-continuous:centos-atomic-host/7/x86_64/devel/continuous

As mentioned in the command output, changes are actually written to an overlayfs:

-bash-4.2# ls /var/tmp/ostree-unlock-ovl.45GXKY/upper/
bin  share  woohoo

These changes are all erased at reboot, though they can be made permanent by using the --hotfix flag. This will make OSTree push back the current deployment as the rollback (so that you can go back to the immutable deployment from which you started) and unlock the current deployment:

-bash-4.2# ostree admin unlock --hotfix
Copying /etc changes: 34 modified, 8 removed, 44650 added
Transaction complete; bootconfig swap: yes deployment count change: 0
Hotfix mode enabled.  A writable overlayfs is now mounted on /usr
for this booted deployment.  A non-hotfixed clone has been created
as the non-default rollback target.
-bash-4.2# ostree admin status
* centos-atomic-host 675583e7db81b59a08e0bbfb3735bddde9d768f9f4500b845799c7e97ab71091.0
    Version: 2016.0.169
    Unlocked: hotfix
    origin refspec: centos-atomic-continuous:centos-atomic-host/7/x86_64/devel/continuous
  centos-atomic-host 675583e7db81b59a08e0bbfb3735bddde9d768f9f4500b845799c7e97ab71091.1
    Version: 2016.0.169
    origin refspec: centos-atomic-continuous:centos-atomic-host/7/x86_64/devel/continuous

Note however, that there can be issues from unlocking due to its use of overlayfs. Most notably, Docker does not currently work with SELinux and OverlayFS (see the related Trello card here).

The other caveat is that a hotfix is only as permanent as its associated deployment. Thus, as soon as you create a new deployment (e.g. through rpm-ostree upgrade/rebase/deploy), the overlayed hotfixed are not carried over, but simply lost.

Package layering in rpm-ostree

This is a relatively much newer feature than system unlocking. It’s part of v2016.4, released 4 days ago. There are updates pending in Fedora’s Bodhi system awaiting your karma. The feature itself has been in the works for a while now, with development starting more than a year ago.

Package layering does exactly what it says; it allows you to extend the base distribution of RPM packages you receive from your content provider with your own packages of choice. In contrast to system unlocking, these additional layered packages are carried over during upgrades, making it great for when your particular system requires an additional package to work correctly.

To use package layering, you can invoke the new pkg-add command of rpm-ostree:

-bash-4.2# rpm-ostree pkg-add strace
notice: pkg-add is a preview command and subject to change.
Need to overlay 1 package onto tree 675583e:

Downloading metadata: [===================================================] 100%
Resolving dependencies... done
Transaction: 1 packages
  strace-4.8-11.el7.x86_64 (base)
Will download: 1 package (271.5 kB)

  Downloading from base: [================================================] 100%

Importing: [==============================================================] 100%
Checking out tree 675583e... done
Overlaying... done
Writing rpmdb... done
Writing OSTree commit... done
Copying /etc changes: 34 modified, 8 removed, 44651 added
Transaction complete; bootconfig swap: no deployment count change: 0
Freed objects: 26.0 MB
Run "systemctl reboot" to start a reboot
-bash-4.2# reboot

Once we’re rebooted, we can make use of the new package:

-bash-4.2# strace -V
strace -- version 4.8

The status output of rpm-ostree will now also reflect this layering (which by the way received a face lift in this release):

-bash-4.2# rpm-ostree status
State: idle
* centos-atomic-continuous:centos-atomic-host/7/x86_64/devel/continuous
       Version: 2016.0.169 (2016-07-12 16:26:00)
    BaseCommit: 675583e7db81b59a08e0bbfb3735bddde9d768f9f4500b845799c7e97ab71091
        Commit: e0291b49619645d4df95c0924a48932f217b0ed4680656ebd47c1b17640a62ee
        OSName: centos-atomic-host
  GPGSignature: (unsigned)
      Packages: strace

       Version: 2016.0.169 (2016-07-11 21:51:17)
        Commit: 675583e7db81b59a08e0bbfb3735bddde9d768f9f4500b845799c7e97ab71091
        OSName: centos-atomic-host
  GPGSignature: (unsigned)

Not surprisingly, packages that were layered using pkg-add can also be later removed by invoking the pkg-remove command, left here as an exercise to the reader.

Under the hood, rpm-ostree takes care of many subtleties, including safely running RPM %post scripts using bubblewrap to ensure that the OSTree system repo remains uncorrupted, and obeying expected RPM rules (e.g. layering a package that conflicts with an existing package will error out).

To summarize, the power that package layering gives you is the ability to customize the package set on particular hosts without having to ship it to all hosts.

Finally, note that package layering is currently in preview mode, which means that it is subject to change, though we will do our best to retain backwards compatibility with layered deployments created with the v2016.4 release.

Bonus section: CentOS Atomic Host Continuous (CAHC)

You might have been wondering what this centos-atomic-continuous remote was about in the command outputs above. The CentOS Atomic SIG has set up an automated job that continuously composes ostree repos using the latest code from the git master of various Project Atomic components, such as atomic, ostree, and rpm-ostree.

This means that within the hour, you can experience the latest and greatest Atomic Host has to offer. For example, if you’d like to try out package layering right away, you can simply rebase onto CAHC and reboot. For more information and instructions, see the devel wiki page.