# 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