By Jake Edge
December 7, 2022

A read-only filesystem that will transparently share file data between disparate
directory trees, while also providing integrity verification for the data
and the
directory metadata, was recently posted as an
RFC
to the linux-kernel mailing list. Composefs was developed
by Alexander Larsson (who posted it) and Giuseppe Scrivano for use by podman containers and OSTree (or “libostree” as it
is now known) root directories, but there are likely others who want the
abilities
it provides. So far, there has been little response, either with feedback or
complaints, but it is a small patch set (around 2K lines of code) and
generally self-contained since it is a filesystem, so it would not be a
surprise to see it appear in some upcoming kernel.

Features

There is a lengthy introduction to composefs in the cover letter and
more information in the documentation
patch. Unlike many filesystems, composefs is not backed by a block
device but instead by a set of regular read-only files: an image file that
contains
the directory structure and file metadata and a directory of
content-addressed objects that are the file contents. Since the files
themselves are in an object store, they are effectively deduplicated at
the file level; files that have identical content are only stored once
even if the metadata (e.g. owner, permissions, extended attr