Filesystem: Physical /sysroot

The bootc project uses ostree as a backend, and maps fetched container images to a deployment.

stateroot

The underlying ostree CLI and API tooling expose a concept of stateroot, which is not yet exposed via bootc. The stateroot used by bootc install is just named default.

The stateroot concept allows having fully separate parallel operating system installations with fully separate /etc and /var, while still sharing an underlying root filesystem.

In the future, this functionality will be exposed and used by bootc.

When booted, the physical root will be available at /sysroot as a read-only mount point. This is a key aspect of how bootc upgrade operates: it fetches the updated container image and writes new files to /sysroot/ostree.

Beyond that and debugging/introspection, there are few use cases for tooling to operate on the physical root.

Expanding the root filesystem

One notable use case that does need to operate on /sysroot is expanding the root filesystem.

Some higher level tools such as e.g. cloud-init may (reasonably) expect the / mount point to be the physical root. Tools like this will need to be adjusted to instead detect this and operate on /sysroot.

Growing the block device

Fundamentally bootc is agnostic to the underlying block device setup. How to grow the root block device depends on the underlying storage stack, from basic partitions to LVM. However, a common tool is the growpart utility from cloud-init.

Growing the filesytem

The systemd project ships a systemd-growfs tool and corresponding systemd-growfs@ services. This is a relatively thin abstraction over detecting the target root filesystem type and running the underlying tool such as xfs_growfs.

At the current time, most Linux filesystems require the target to be mounted writable in order to grow. Hence, an invocation of system-growfs /sysroot or xfs_growfs /sysroot will need to be further wrapped in a temporary mount namespace.

Using a MountFlags=slave drop-in stanza for systemd-growfs@sysroot.service is recommended, along with an ExecStartPre=mount -o remount,rw /sysroot.

Detecting bootc/ostree systems

For tools like cloud-init that want to operate generically, conditionally detecting this situation can be done via e.g.:

Checking for / being an overlay mount point
Checking for /sysroot/ostree

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