ostree/docs/atomic-upgrades.md

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Atomic Upgrades

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You can turn off the power anytime you want...

OSTree is designed to implement fully atomic and safe upgrades; more generally, atomic transitions between lists of bootable deployments. If the system crashes or you pull the power, you will have either the old system, or the new one.

Simple upgrades via HTTP

First, the most basic model OSTree supports is one where it replicates pre-generated filesystem trees from a server over HTTP, tracking exactly one ref, which is stored in the .origin file for the deployment. The command ostree admin upgrade implements this.

To begin a simple upgrade, OSTree fetches the contents of the ref from the remote server. Suppose we're tracking a ref named exampleos/buildmain/x86_64-runtime. OSTree fetches the URL http://example.com/repo/refs/heads/exampleos/buildmain/x86_64-runtime, which contains a SHA256 checksum. This determines the tree to deploy, and /etc will be merged from currently booted tree.

If we do not have this commit, then we perform a pull process. At present (without static deltas), this involves quite simply just fetching each individual object that we do not have, asynchronously. Put in other words, we only download changed files (zlib-compressed). Each object has its checksum validated and is stored in /ostree/repo/objects/.

Once the pull is complete, we have downloaded all the objects that we need to perform a deployment.

Upgrades via external tools (e.g. package managers)

As mentioned in the introduction, OSTree is also designed to allow a model where filesystem trees are computed on the client. It is completely agnostic as to how those trees are generated; they could be computed with traditional packages, packages with post-deployment scripts on top, or built by developers directly from revision control locally, etc.

At a practical level, most package managers today (dpkg and rpm) operate "live" on the currently booted filesystem. The way they could work with OSTree is to, instead, take the list of installed packages in the currently booted tree, and compute a new filesystem from that. A later chapter describes in more details how this could work: Adapting Existing Systems.

For the purposes of this section, let's assume that we have a newly generated filesystem tree stored in the repo (which shares storage with the existing booted tree). We can then move on to checking it back out of the repo into a deployment.

Assembling a new deployment directory

Given a commit to deploy, OSTree first allocates a directory for it. This is of the form /boot/loader/entries/ostree-$stateroot-$checksum.$serial.conf. The $serial is normally 0, but if a given commit is deployed more than once, it will be incremented. This is supported because the previous deployment may have configuration in /etc that we do not want to use or overwrite.

Now that we have a deployment directory, a 3-way merge is performed between the (by default) currently booted deployment's /etc, its default configuration, and the new deployment (based on its /usr/etc).

How it works is:

• Files in the currently booted deployment's /etc which were modified from the default /usr/etc (of the same deployment) are retained.
• Files in the currently booted deployment's /etc which were not modified from the default /usr/etc (of the same deployment) are upgraded to the new defaults from the new deployment's /usr/etc.

Roughly, this means that as soon as you modify or add a file in /etc, this file will be propagated forever as is (though there is a corner-case, where if your modification eventually exactly matches a future default file, then the file will go back to following future default updates from that point on).

You can use ostree admin config-diff to see the differences between your booted deployment's /etc and the OSTree defaults. A command like diff {/usr,}/etc will additional print line-level differences.

Atomically swapping boot configuration

At this point, a new deployment directory has been created as a hardlink farm; the running system is untouched, and the bootloader configuration is untouched. We want to add this deployment to the "deployment list".

To support a more general case, OSTree supports atomic transitioning between arbitrary sets of deployments, with the restriction that the currently booted deployment must always be in the new set. In the normal case, we have exactly one deployment, which is the booted one, and we want to add the new deployment to the list. A more complex command might allow creating 100 deployments as part of one atomic transaction, so that one can set up an automated system to bisect across them.

The bootversion

OSTree allows swapping between boot configurations by implementing the "swapped directory pattern" in /boot. This means it is a symbolic link to one of two directories /ostree/boot.[0|1]. To swap the contents atomically, if the current version is 0, we create /ostree/boot.1, populate it with the new contents, then atomically swap the symbolic link. Finally, the old contents can be garbage collected at any point.

The /ostree/boot directory

However, we want to optimize for the case where the set of kernel/initramfs/devicetree sets is the same between both the old and new deployment lists. This happens when doing an upgrade that does not include the kernel; think of a simple translation update. OSTree optimizes for this case because on some systems /boot may be on a separate medium such as flash storage not optimized for significant amounts of write traffic. Related to this, modern OSTree has support for having /boot be a read-only mount by default - it will automatically remount read-write just for the portion of time necessary to update the bootloader configuration.

To implement this, OSTree also maintains the directory /ostree/boot.$bootversion, which is a set of symbolic links to the deployment directories. The $bootversion here must match the version of /boot. However, in order to allow atomic transitions of this directory, this is also a swapped directory, so just like /boot, it has a version of 0 or 1 appended.

Each bootloader entry has a special ostree= argument which refers to one of these symbolic links. This is parsed at runtime in the initramfs.