Built on top of the OSTree versioning filesystem core is a layer that knows how to deploy, parallel install, and manage Unix-like operating systems (accessible via ostree admin). The core content of these operating systems are treated as read-only, but they transparently share storage.

A deployment is physically located at a path of the form /ostree/deploy/$osname/deploy/$checksum. OSTree is designed to boot directly into exactly one deployment at a time; each deployment is intended to be a target for chroot() or equivalent.

"osname": Group of deployments that share /var

Each deployment is grouped in exactly one "osname". From above, you can see that an osname is physically represented in the /ostree/deploy/$osname directory. For example, OSTree can allow parallel installing Debian in /ostree/deploy/debian and Red Hat Enterprise Linux in /ostree/deploy/rhel (subject to operating system support, present released versions of these operating systems may not support this).

Each osname has exactly one copy of the traditional Unix /var, stored physically in /ostree/deploy/$osname/var. OSTree provides support tools for systemd to create a Linux bind mount that ensures the booted deployment sees the shared copy of /var.

OSTree does not touch the contents of /var. Operating system components such as daemon services are required to create any directories they require there at runtime (e.g. /var/cache/$daemonname), and to manage upgrading data formats inside those directories.

Contents of a deployment

A deployment begins with a specific commit (represented as a SHA256 hash) in the OSTree repository in /ostree/repo. This commit refers to a filesystem tree that represents the underlying basis of a deployment. For short, we will call this the "tree", to distinguish it from the concept of a deployment.

First, the tree must include a kernel stored as vmlinuz(-.*)?-$checksum in either /boot or /usr/lib/ostree-boot. The checksum should be a SHA256 hash of the kernel contents; it must be pre-computed before storing the kernel in the repository. Optionally, the directory can also contain an initramfs, stored as initramfs(-.*)?-$checksum. If this exists, the checksum must include both the kernel and initramfs contents. OSTree will use this to determine which kernels are shared. The rationale for this is to avoid computing checksums on the client by default.

The deployment should not have a traditional UNIX /etc; instead, it should include /usr/etc. This is the "default configuration". When OSTree creates a deployment, it performs a 3-way merge using the old default configuration, the active system's /etc, and the new default configuration. In the final filesystem tree for a deployment then, /etc is a regular writable directory.

Besides the exceptions of /var and /etc then, the rest of the contents of the tree are checked out as hard links into the repository. It's strongly recommended that operating systems ship all of their content in /usr, but this is not a hard requirement.

Finally, a deployment may have a .origin file, stored next to its directory. This file tells ostree admin upgrade how to upgrade it. At the moment, OSTree only supports upgrading a single refspec. However, in the future OSTree may support a syntax for composing layers of trees, for example.

The system /boot

While OSTree parallel installs deployments cleanly inside the /ostree directory, ultimately it has to control the system's /boot directory. The way this works is via the Boot Loader Specification, which is a standard for bootloader-independent drop-in configuration files.

When a tree is deployed, it will have a configuration file generated of the form /boot/loader/entries/ostree-$osname-$checksum.$serial.conf. This configuration file will include a special ostree= kernel argument that allows the initramfs to find (and chroot() into) the specified deployment.

At present, not all bootloaders implement the BootLoaderSpec, so OSTree contains code for some of these to regenerate native config files (such as /boot/syslinux/syslinux.conf) based on the entries.