Alexander Larsson, creator of Flatpak and working at Red Hat, presented a preliminary version of patches implementing the Composefs filesystem for the Linux kernel. The proposed filesystem is similar to Squashfs and is also suitable for mounting images in read-only mode. The differences boil down to providing efficient sharing of multiple mounted disk images' content and supporting the authenticity verification of read data. Areas of application where Composefs may be in demand include mounting container images and its use for Git-like OSTree repositories.
Composefs uses a content-addressable storage model, meaning that the primary identifier is not the filename, but a hash of the file's contents. This model ensures deduplication, allowing only one copy of identical files found across different mounted partitions to be stored. For instance, container images contain many standard system files, and when using Composefs, each of these files will be shared by all mounted images without tricks like hard link forwarding. Common files are stored as a single copy on disk and accounted for as one entry in the page cache, which helps save both disk and RAM.
To save disk space, the data and metadata in mounted images are separated. When mounting, the following are specified:
- A binary index containing all filesystem metadata, filenames, permissions, and other information, excluding the actual file contents.
- A base directory where the content of all mounted images’ files is stored. Files are stored linked to a hash of their contents.
Binarý index sa vytvára pre každú obrazovú FS, zatiaľ čo základný adresár je spoločný pre všetky obrazy. Na overenie obsahu jednotlivých súborov a celého obrazu v podmienkach zdieľaného úložiska môže byť použitý mechanizmus fs-verity, ktorý pri prístupe k súborom kontroluje zhody uvedených v binárnom indexe hashov s faktickým obsahom (t.j. ak útočník zmení súbor v základnom adresári alebo sa dáta poškodia v dôsledku zlyhania, takáto kontrola odhalí nesúlad).
Zdroj: opennet.ru
