55603a0c52
I never really liked the term "osname". I feel "stateroot" is a *lot* clearer, since the osname/stateroot mostly just holds `/var`. Further it avoids the `os` prefix which is already overloaded. Some of the existing docs already talked about "operating system state", which further reinforces this. There's *lot* more things than this which reference the term "osname", but I don't want to change *everything* yet in this patch in case we decide to do something different - this just gets the highlights. Closes: #794 Approved by: jlebon
4.3 KiB
Deployments
Overview
Built on top of the OSTree versioning filesystem core is a layer
that knows how to deploy, parallel install, and manage Unix-like
operating systems (accessible via ostree admin). The core content of these operating systems
are treated as read-only, but they transparently share storage.
A deployment is physically located at a path of the form
/ostree/deploy/$stateroot/deploy/$checksum.
OSTree is designed to boot directly into exactly one deployment
at a time; each deployment is intended to be a target for
chroot() or equivalent.
"stateroot" (AKA "osname"): Group of deployments that share /var
Each deployment is grouped in exactly one "stateroot" (also known as an "osname"); the former term is preferred.
From above, you can see that an stateroot is physically represented in the
/ostree/deploy/$stateroot directory. For example, OSTree can allow parallel
installing Debian in /ostree/deploy/debian and Red Hat Enterprise Linux in
/ostree/deploy/rhel (subject to operating system support, present released
versions of these operating systems may not support this).
Each stateroot has exactly one copy of the traditional Unix /var,
stored physically in /ostree/deploy/$stateroot/var. OSTree provides
support tools for systemd to create a Linux bind mount that ensures
the booted deployment sees the shared copy of /var.
OSTree does not touch the contents of /var. Operating system
components such as daemon services are required to create any
directories they require there at runtime
(e.g. /var/cache/$daemonname), and to manage upgrading data formats
inside those directories.
Contents of a deployment
A deployment begins with a specific commit (represented as a
SHA256 hash) in the OSTree repository in /ostree/repo. This commit refers
to a filesystem tree that represents the underlying basis of a
deployment. For short, we will call this the "tree", to
distinguish it from the concept of a deployment.
First, the tree must include a kernel stored as
vmlinuz(-.*)?-$checksum in either /boot or /usr/lib/ostree-boot.
The checksum should be a SHA256 hash of the kernel contents; it must be
pre-computed before storing the kernel in the repository. Optionally,
the directory can also contain an initramfs, stored as
initramfs(-.*)?-$checksum. If this exists, the checksum must include
both the kernel and initramfs contents. OSTree will use this to
determine which kernels are shared. The rationale for this is to avoid
computing checksums on the client by default.
The deployment should not have a traditional UNIX /etc; instead, it
should include /usr/etc. This is the "default configuration". When
OSTree creates a deployment, it performs a 3-way merge using the
old default configuration, the active system's /etc, and the new
default configuration. In the final filesystem tree for a deployment
then, /etc is a regular writable directory.
Besides the exceptions of /var and /etc then, the rest of the
contents of the tree are checked out as hard links into the
repository. It's strongly recommended that operating systems ship all
of their content in /usr, but this is not a hard requirement.
Finally, a deployment may have a .origin file, stored next to its
directory. This file tells ostree admin upgrade how to upgrade it.
At the moment, OSTree only supports upgrading a single refspec.
However, in the future OSTree may support a syntax for composing
layers of trees, for example.
The system /boot
While OSTree parallel installs deployments cleanly inside the
/ostree directory, ultimately it has to control the system's /boot
directory. The way this works is via the
Boot Loader Specification,
which is a standard for bootloader-independent drop-in configuration
files.
When a tree is deployed, it will have a configuration file generated
of the form
/boot/loader/entries/ostree-$stateroot-$checksum.$serial.conf. This
configuration file will include a special ostree= kernel argument
that allows the initramfs to find (and chroot() into) the specified
deployment.
At present, not all bootloaders implement the BootLoaderSpec, so
OSTree contains code for some of these to regenerate native config
files (such as /boot/syslinux/syslinux.conf) based on the entries.