Crate composefs 

composefs: The reliability of disk images, the flexibility of files

composefs combines several Linux kernel features to provide read-only mountable filesystem trees that stack on top of a conventional “lower” filesystem.

Interfaces

composefs offers two programmatic interfaces:

• Rust API — this crate and its siblings (composefs-oci, composefs-boot, etc.), usable as regular Cargo dependencies.
• Varlink API — a varlink RPC interface exposed by cfsctl varlink over a Unix socket, accessible from any language. See the varlink module for examples.

Neither interface is declared stable yet. Both may change across releases while the project is under active development.

Key technologies

• overlayfs — the kernel mount interface that exposes the composed tree
• EROFS — an in-kernel read-only filesystem for the metadata tree (directories, symlinks, permissions, xattrs) with no file data
• fs-verity (optional) — per-file integrity verification on the backing store, validated by overlayfs at access time

Design

composefs produces an EROFS image containing only metadata. Non-empty data files live in a content-addressed backing store, with trusted.overlay.redirect xattrs telling overlayfs where to find them. Identical files across images are stored once on disk and shared in the Linux page cache.

See the repository_format module for the on-disk layout.

Re-exports

pub use repository::ImageNotFound;

Modules

digest

Cryptographic hash digest wrappers backed by OpenSSL.

dumpfile

Reading and writing composefs dumpfile format.

dumpfile_parse

Parsing and generating composefs dump file entry

erofs

EROFS (Enhanced Read-Only File System) format support for composefs.

erofs_format

composefs EROFS image format

filesystem_ops

High-level filesystem operations for composefs trees.

fs

Reading and writing filesystem trees to/from disk.

fsverity

Linux fs-verity support for integrity verification.

generic_tree

A generic metadata-only filesystem tree where regular files can be stored however the caller wants.

mount

Modern Linux mount API support for composefs.

mountcompat

Compatibility helpers for older Linux kernel mount APIs.

progress

Progress reporting API for pull and download operations.

repository

Content-addressable repository for composefs objects.

repository_format

composefs repository design

splitstream

Split Stream file format implementation.

splitstream_format

Splitstream

tree

A filesystem tree which stores regular files using the composefs strategy of inlining small files, and having an external fsverity reference for larger ones.

util

Utility functions and types used throughout the composefs crate.

varlink

Varlink API

Constants

INLINE_CONTENT_MAX_V0

Files with this many bytes or fewer are stored inline in the erofs image (and in splitstreams). Files above this threshold are written to object storage and referenced via overlay metacopy xattrs.

MAX_INLINE_CONTENT

Maximum inline content size accepted when parsing untrusted input (dumpfiles, EROFS images in composefs-restricted mode).

SYMLINK_MAX

Maximum symlink target length in bytes.