# What is OSTree and RPM-OSTree? OSTree is a tool for managing bootable, immutable, versioned filesystem trees. ## Photon RPM-OSTree Server During installation from the Photon OS ISO, you can select the `Photon RPM-OSTree Server` option to install Photon OS with the RPM-OSTree components and a minimal repository from which RPM-OSTree Hosts can be created. On RPM-OSTree Server the repo is created at the following location: ``` /srv/rpm-ostree/repo ``` Here is the base configuration file that creates the repo: ``` /srv/rpm-ostree/photon-base.json ``` ### Updating RPM-OSTree Repo Image You can add or remove packages from the `photon-base.json` file and then run following commands to update the repo with a new commit: ``` cd /srv/rpm-ostree rpm-ostree compose tree --repo=/srv/rpm-ostree/repo photon-base.json ``` The command above creates a new commit that contains your package-related changes. ## RPM-OSTree Host OSTree host is an installation of Photon OS that retrieves the images from RPM-OSTree server during installation. It can get atomic updates from the same server during its lifecycle. ## Upgrading the Host Installations Host machines created from RPM-OSTree server can run the following commands to upgrade their host machine to the latest version from the RPM-OSTree server: rpm-ostree upgrade systemctl reboot