OSTree for Fedora

Linux distributions are typically delivered either in piecemeal fashion via packages or as a single bundle like Android or Chrome OS. Colin Walters's OSTree project is taking something of a middle course. It is delivering a kind of bundle, but with a twist: the tool makes it easy to switch between multiple independent operating systems that are all simultaneously installed on the system.

We first looked at OSTree as part of our coverage of GUADEC 2012. Since that talk, though, Walters has not been idle. His gnome-continuous project has been building OSTree versions of GNOME using continuous integration for some time. Beyond that, he recently announced an OSTree repository for Fedora: fedostree:

Rather than break things up into packages, OSTree builds fully assembled, versioned filesystem trees that hold what would have gone into a set of package (e.g. RPM) files. All of the binaries, libraries, documentation files, and so on are gathered together into the OSTree repository and can be retrieved using the ostree command. The fedostree installation instructions make it clear that the model Walters used was Git; many of the commands are clearly patterned after Git's. In fact, the project's home page calls it "git for operating system binaries".

For Fedora, one can install multiple different kinds of trees, based on either Fedora 20 or Rawhide, and switch between them at boot time. Each of the trees will have a focus on a particular project's output, such as GNOME, Docker, FreeIPA, or others. Those can be combined with a tree that provides the minimal F20 or Rawhide base to produce a system that can boot and run the project's output, and can do so repeatedly as each new version gets built from newer upstream code. Developers on those projects will then have a tool to quickly and easily switch between multiple versions of the project. That can make bisecting problems much easier, for example. Beyond the project documentation and Walters's announcement, he also describes some of the details of the tree switching in a blog post.

Each of the OSTree-installed systems will share the /etc directory; changes will be merged with those made by the other OSTrees. Also, /var will be shared between the instances. Beyond that, OSTree does quite a bit of filesystem rearranging to enable it to switch between the different trees. In the blog post, Walters gives some details:

# ostree admin switch fedostree/20/x86_64/server/docker-io

# ostree admin switch fedostree/20/workstation/gnome/core

In addition to bisecting problems in broken umbrella projects (like GNOME or Mesa) that may not be easy to find otherwise, there are a number of other possible uses that Walters lists on the fedostree page. Installing RHEL and Fedora in parallel, or Fedora and Rawhide, would be one possibility. That would make switching between them far easier, while still keeping each up to date. It would also make testing Rawhide much safer since the only real risk is a filesystem-corrupting kernel bug. Making an Android-like recovery partition with a minimal install is another idea.

As he notes in the announcement (and elsewhere), fedostree (and OSTree itself) are rather "bleeding edge" at this point. He is looking for brave souls to evaluate the ideas, tools, and repositories to help him find problems. There is also a list of "development areas" that need to be addressed in the near future.

OSTree/fedostree is clearly a project that is just getting off the ground. OSTree has proven itself useful for GNOME continuous integration, so Walters plans to bring that to fedostree; trees would get rebuilt (and booted as a smoketest) immediately after a change is made that affects any package relevant to that tree. Before much of that can happen, though, some testing and shaking things out is required. It should be interesting to see where things go from here.