## 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 `/boot/vmlinuz-`checksum``. 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 tree can contain an initramfs, stored as `/boot/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.