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Adapting existing mainstream distributions

  1. System layout
  2. Booting and initramfs technology
  3. /usr/lib/passwd
  4. Adapting existing package managers
    1. Licensing for this document:

System layout

First, OSTree encourages systems to implement UsrMove This is simply to avoid the need for more bind mounts. By default OSTree’s dracut hook creates a read-only bind mount over /usr; you can of course generate individual bind-mounts for /bin, all the /lib variants, etc. So it is not intended to be a hard requirement.

Remember, because by default the system is booted into a chroot equivalent, there has to be some way to refer to the actual physical root filesystem. Therefore, your operating system tree should contain an empty /sysroot directory; at boot time, OSTree will make this a bind mount to the physical / root directory. There is precedent for this name in the initramfs context. You should furthermore make a toplevel symbolic link /ostree which points to /sysroot/ostree, so that the OSTree tool at runtime can consistently find the system data regardless of whether it’s operating on a physical root or inside a deployment.

Because OSTree only preserves /var across upgrades (each deployment’s chroot directory will be garbage collected eventually), you will need to choose how to handle other toplevel writable directories specified by the Filesystem Hierarchy Standard. Your operating system may of course choose not to support some of these such as /usr/local, but following is the recommended set:

  • /home/var/home
  • /opt/var/opt
  • /srv/var/srv
  • /root/var/roothome
  • /usr/local/var/usrlocal
  • /mnt/var/mnt
  • /tmp/sysroot/tmp

Furthermore, since /var is empty by default, your operating system will need to dynamically create the targets of these at boot. A good way to do this is using systemd-tmpfiles, if your OS uses systemd. For example:

d /var/log/journal 0755 root root -
L /var/home - - - - ../sysroot/home
d /var/opt 0755 root root -
d /var/srv 0755 root root -
d /var/roothome 0700 root root -
d /var/usrlocal 0755 root root -
d /var/usrlocal/bin 0755 root root -
d /var/usrlocal/etc 0755 root root -
d /var/usrlocal/games 0755 root root -
d /var/usrlocal/include 0755 root root -
d /var/usrlocal/lib 0755 root root -
d /var/usrlocal/man 0755 root root -
d /var/usrlocal/sbin 0755 root root -
d /var/usrlocal/share 0755 root root -
d /var/usrlocal/src 0755 root root -
d /var/mnt 0755 root root -
d /run/media 0755 root root -

Particularly note here the double indirection of /home. By default, each deployment will share the global toplevel /home directory on the physical root filesystem. It is then up to higher levels of management tools to keep /etc/passwd or equivalent synchronized between operating systems. Each deployment can easily be reconfigured to have its own home directory set simply by making /var/home a real directory.

Booting and initramfs technology

OSTree comes with optional dracut+systemd integration code which follows this logic:

  • Parse the ostree= kernel command line argument in the initramfs
  • Set up a read-only bind mount on /usr
  • Bind mount the deployment’s /sysroot to the physical /
  • Use mount(MS_MOVE) to make the deployment root appear to be the root filesystem

After these steps, systemd switches root.

If you are not using dracut or systemd, using OSTree should still be possible, but you will have to write the integration code. See the existing sources in src/switchroot as a reference.

Patches to support other initramfs technologies and init systems, if sufficiently clean, will likely be accepted upstream.

A further specific note regarding sysvinit: OSTree used to support recording device files such as the /dev/initctl FIFO, but no longer does. It’s recommended to just patch your initramfs to create this at boot.


Unlike traditional package systems, OSTree trees contain numeric uid and gids. Furthermore, it does not have a %post type mechanism where useradd could be invoked. In order to ship an OS that contains both system users and users dynamically created on client machines, you will need to choose a solution for /etc/passwd. The core problem is that if you add a user to the system for a daemon, the OSTree upgrade process for /etc will simply notice that because /etc/passwd differs from the previous default, it will keep the modified config file, and your new OS user will not be visible. The solution chosen for the Gnome Continuous operating system is to create /usr/lib/passwd, and to include a NSS module nss-altfiles which instructs glibc to read from it. Then, the build system places all system users there, freeing up /etc/passwd to be purely a database of local users. See also a more recent effort from Systemd Stateless

Adapting existing package managers

The largest endeavor is likely to be redesigning your distribution’s package manager to be on top of OSTree, particularly if you want to keep compatibility with the “old way” of installing into the physical /. This section will use examples from both dpkg and rpm as the author has familiarity with both; but the abstract concepts should apply to most traditional package managers.

There are many levels of possible integration; initially, we will describe the most naive implementation which is the simplest but also the least efficient. We will assume here that the admin is booted into an OSTree-enabled system, and wants to add a set of packages.

Many package managers store their state in /var; but since in the OSTree model that directory is shared between independent versions, the package database must first be found in the per-deployment /usr directory. It becomes read-only; remember, all upgrades involve constructing a new filesystem tree, so your package manager will also need to create a copy of its database. Most likely, if you want to continue supporting non-OSTree deployments, simply have your package manager fall back to the legacy /var location if the one in /usr is not found.

To install a set of new packages (without removing any existing ones), enumerate the set of packages in the currently booted deployment, and perform dependency resolution to compute the complete set of new packages. Download and unpack these new packages to a temporary directory.

Now, because we are merely installing new packages and not removing anything, we can make the major optimization of reusing our existing filesystem tree, and merely layering the composed filesystem tree of these new packages on top. A command like this:

ostree commit -b osname/releasename/description

will create a new commit in the $osname/$releasename/$description branch. The OSTree SHA256 checksum of all the files in /var/tmp/newpackages.13A8D0/ will be computed, but we will not re-checksum the present existing tree. In this layering model, earlier directories will take precedence, but files in later layers will silently override earlier layers.

Then to actually deploy this tree for the next boot: ostree admin deploy $osname/$releasename/$description

This is essentially what rpm-ostree does to support its package layering model.

Licensing for this document:

SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later)