For the last couple of weeks, I’ve been working on garbage collection for composefs repositories. A composefs repository is a content-addressed object store, and it needs a garbage collector to reclaim space from images that are no longer referenced. The hard part is deciding what “referenced” actually means when an image has no ref in the repository but is mounted somewhere, possibly from another process, possibly in a different mount namespace, possibly by the running system itself. On composefs-rs#346 was suggested to use flock() on the EROFS backing file. Mounters take a shared lock, the GC probes with an exclusive one, and if the exclusive lock fails, the image is still in use.

Composefs achieves whole-filesystem integrity verification through image sealing: a single cryptographic digest authenticates an entire filesystem, covering both file contents and metadata (directory structure, permissions, ownership, symlinks, and xattrs). This goes further than fs-verity alone, which can only verify individual file contents, and avoids the fixed-partition requirement of dm-verity. The mechanism combines an EROFS image for metadata, a content-addressed object store for file data, and overlayfs with verity=require to enforce integrity checks on every file access at the kernel level.

For the last couple of weeks, I’ve been playing on a PoC implementation of a file system for the Linux kernel. The goal is to address a fundamental limitation in how container images are stored: the existing overlay model deduplicates at the layer level, but once you want per-file deduplication — so that identical files across different images share a single copy on disk and in the page cache — the current architecture gets in the way and requires awkward workarounds involving hard links or filesystem-specific reflinks.