History
Origin
composefs came out of Red Hat, driven by Alexander Larsson (the author of Flatpak) and Giuseppe Scrivano. The goal was per-file sharing plus runtime verification for two existing systems: the container layers used by podman and the OSTree (libostree) object store used for bootable hosts. The repository on GitHub was created on 2021-10-07 (source 2). The first public proposal was a kernel mailing-list RFC on 2022-11-28, covered by LWN as "an opportunistically sharing verified image filesystem" and by Phoronix (source 5, source 10).
The original RFC proposed composefs as a standalone read-only kernel filesystem module. The kernel community pushed back: rather than a new filesystem, the same result could be had by combining the overlayfs, EROFS, and fs-verity features that already existed. composefs pivoted to that composition design (source 6, source 8).
Timeline
| Year | Milestone |
|---|---|
| 2021 | Repository created on GitHub (2021-10-07) (source 2). |
| 2022 | First public kernel RFC for a standalone composefs filesystem (2022-11-28) (source 5). |
| 2023 | Pivot to overlayfs + EROFS + fs-verity composition; the required overlayfs fs-verity support landed in Linux 6.6-rc1, and composefs 1.0 shipped on 2023-09-26 (source 6, source 7). |
| 2025 | Accepted as a CNCF Sandbox project on 2025-01-21 (source 3, source 4). |
How it evolved
The defining shift was abandoning the standalone-filesystem approach. Once the maintainers accepted the kernel community's verdict, the work moved into two places: getting the missing piece (fs-verity support in overlayfs) merged upstream, and stabilizing the on-disk image format on the composefs side. With overlayfs fs-verity support in Linux 6.6-rc1, every kernel change composefs needed was upstream, so the project froze its image format and cut 1.0 (source 7).
The earlier exploration is visible in the OSTree integration writeup, which described mounting a composefs image that points into the OSTree object store instead of checking files out into a regular directory (source 9, tracked in source 11).
Where it stands now
composefs is a CNCF Sandbox project as of 2025-01-21 (source 3). At the commit documented here (298edd6, 2026-06-12) the meson.build version is 1.0.8 (src/meson.build:4), and the documented commit sits on main after that tag. The codebase is a focused C library (liblcfs) plus a handful of command-line tools, and Alexander Larsson remains the dominant contributor (609 commits per the GitHub contributors API, source 2). The stated direction is to serve as a backing format for container and bootable-host systems rather than to grow into a general-purpose filesystem.