OSTree Overview

Introduction

OSTree 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, and 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.

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.