OSTree Overview

Introduction

OSTree is an upgrade system for Linux-based operating systems that performs atomic upgrades of complete filesystem trees. It is not a package system; rather, it is intended to complement them. A primary model is composing packages on a server, and then replicating them to clients.

The underlying architecture might be summarized as "git for operating system binaries". It operates in userspace, and will work on top of any Linux filesystem. At its core is a git-like content-addressed object store with branches (or "refs") to track meaningful filesystem trees within the store. Similarly, one can check out or commit to these branches.

Layered on top of that is bootloader configuration, management of /etc, and other functions to perform an upgrade beyond just replicating files.

You can use OSTree standalone in the pure replication model, but another approach is to add a package manager on top, thus creating a hybrid tree/package system.

Hello World example

OSTree is mostly used as a library, but a quick tour of using its CLI tools can give a general idea of how it works at its most basic level.

You can create a new OSTree repository using init:

$ ostree --repo=repo init

This will create a new repo directory containing your repository. Feel free to inspect it.

Now, let's prepare some data to add to the repo:

$ mkdir tree
$ echo "Hello world!" > tree/hello.txt

We can now import our tree/ directory using the commit command:

$ ostree --repo=repo commit --branch=foo tree/

This will create a new branch foo pointing to the full tree imported from tree/. In fact, we could now delete tree/ if we wanted to.

To check that we indeed now have a foo branch, you can use the refs command:

$ ostree --repo=repo refs
foo

We can also inspect the filesystem tree using the ls and cat commands:

$ ostree --repo=repo ls foo
d00775 1000 1000      0 /
-00664 1000 1000     13 /hello.txt
$ ostree --repo=repo cat foo /hello.txt
Hello world!

And finally, we can check out our tree from the repository:

$ ostree --repo=repo checkout foo tree-checkout/
$ cat tree-checkout/hello.txt
Hello world!

Comparison with "package managers"

Because OSTree is designed for deploying core operating systems, a comparison with traditional "package managers" such as dpkg and rpm is illustrative. Packages are traditionally composed of partial filesystem trees with metadata and scripts attached, and these are dynamically assembled on the client machine, after a process of dependency resolution.

In contrast, OSTree only supports recording and deploying complete (bootable) filesystem trees. It has no built-in knowledge of how a given filesystem tree was generated or the origin of individual files, or dependencies, descriptions of individual components. Put another way, OSTree only handles delivery and deployment; you will likely still want to include inside each tree metadata about the individual components that went into the tree. For example, a system administrator may want to know what version of OpenSSL was included in your tree, so you should support the equivalent of rpm -q or dpkg -L.

The OSTree core emphasizes replicating read-only OS trees via HTTP, and where the OS includes (if desired) an entirely separate mechanism to install applications, stored in /var if they're system global, or /home for per-user application installation. An example application mechanism is http://docker.io/

However, it is entirely possible to use OSTree underneath a package system, where the contents of /usr are computed on the client. For example, when installing a package, rather than changing the currently running filesystem, the package manager could assemble a new filesystem tree that layers the new packages on top of a base tree, record it in the local OSTree repository, and then set it up for the next boot. To support this model, OSTree provides an (introspectable) C shared library.

Comparison with block/image replication

OSTree shares some similarity with "dumb" replication and stateless deployments, such as the model common in "cloud" deployments where nodes are booted from an (effectively) readonly disk, and user data is kept on a different volumes. The advantage of "dumb" replication, shared by both OSTree and the cloud model, is that it's reliable and predictable.

But unlike many default image-based deployments, OSTree supports exactly two persistent writable directories that are preserved across upgrades: /etc and /var.

Because OSTree operates at the Unix filesystem layer, it works on top of any filesystem or block storage layout; it's possible to replicate a given filesystem tree from an OSTree repository into plain ext4, BTRFS, XFS, or in general any Unix-compatible filesystem that supports hard links. Note: OSTree will transparently take advantage of some BTRFS features if deployed on it.

OSTree is orthogonal to virtualization mechanisms like AMIs and qcow2 images, though it's most useful though if you plan to update stateful VMs in-place, rather than generating new images.

In practice, users of "bare metal" configurations will find the OSTree model most useful.

Atomic transitions between parallel-installable read-only filesystem trees

Another deeply fundamental difference between both package managers and image-based replication is that OSTree is designed to parallel-install multiple versions of multiple independent operating systems. OSTree relies on a new toplevel ostree directory; it can in fact parallel install inside an existing OS or distribution occupying the physical / root.

On each client machine, there is an OSTree repository stored in /ostree/repo, and a set of "deployments" stored in /ostree/deploy/$STATEROOT/$CHECKSUM. Each deployment is primarily composed of a set of hardlinks into the repository. This means each version is deduplicated; an upgrade process only costs disk space proportional to the new files, plus some constant overhead.

The model OSTree emphasizes is that the OS read-only content is kept in the classic Unix /usr; it comes with code to create a Linux read-only bind mount to prevent inadvertent corruption. There is exactly one /var writable directory shared between each deployment for a given OS. The OSTree core code does not touch content in this directory; it is up to the code in each operating system for how to manage and upgrade state.

Finally, each deployment has its own writable copy of the configuration store /etc. On upgrade, OSTree will perform a basic 3-way diff, and apply any local changes to the new copy, while leaving the old untouched.

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