# What is OSTree and RPM-OSTree
OSTree is a tool for managing bootable, immutable, versioned filesystem trees.
## Photon RPM-OSTree Server
During installation from Photon ISO you can select "Photon RPM-OSTree Server" option that will install Photon with RPM-OSTree components and a minimal repo from which RPM-OSTree Hosts could be created.
On RPM-OSTree Server the repo is created at following location.
```
/srv/rpm-ostree/repo
```
Following is the base configuration file used to create the repo.
```
/srv/rpm-ostree/photon-base.json
```
### Updating RPM-OSTree repo image
You can add/remove package from photon-base.json file and then run following command to update the repo with new commit.
```
cd /srv/rpm-ostree
rpm-ostree compose tree --repo=/srv/rpm-ostree/repo photon-base.json
```
Above command will create a new commit that will have your package related changes in it.
## RPM-OSTree Host
OSTree host is the installation of Photon that retrieves the images from RPM-OSTree server during installation. It can get atomic updates from the same server in its life time.
## Upgrading the Host installations
Host machines created from RPM-OSTree server can run following commands to upgrade their Host machine to the latest version from RPM-OSTree server.
```
> rpm-ostree upgrade
> systemctl reboot
```