A demo fs for learning from bcachefs.

Backed by a single image file (default mode is still in-memory; pass --image <path> to mount on a persistent file). Synchronous writes only — no journal, no crash recovery yet.

Features

Core storage:

• COW B-tree with split / rebalance, multi-bset node layout (k-way merged sorted runs)
• Zerocopy on-disk layout (4 KB nodes, NodeHeader / DiskEntry)
• Multi-tree view via key prefix (inode / dirent / extent in one physical btree, bcachefs style)
• In-place delete optimization (flip kind byte when deleting own-snap key)

Snapshots:

• snap_id embedded in every key, iterator ancestor filtering
• Snapshot tree + Subvolume tree
• Writable snapshots via snapshot_subvol / switch_subvol

FUSE (fuser 0.17, pure Rust):

• lookup / getattr / readdir / read / write / create / mkdir / unlink / rmdir / rename
• Multi-block writes (4 KB chunks) and zero-filled sparse reads
• Atomic multi-key transactions for metadata ops

Persistence (phase 1):

• Single backing image file with superblock, CRC32 per node block
• BlockStore with append-only FrozenMap cache (borrow-stable across faults)
• Fs::create / Fs::open / Fs::sync; FUSE auto-syncs on destroy

TODO

Near-term:

• needs_whiteout bit + whiteout-only compaction
• Snapshot deletion (walk btrees, drop gone snap_id keys, clean whiteouts)
• Sibling merge / rebalance on sparse leaves
• setattr: truncate / chmod / utimens
• Block reclaim on overwrite / unlink (needs per-block refcounting with snapshots)
• Expose subvolume management via FUSE

Persistence phase 2:

• Write-ahead journal + crash recovery
• Block GC (mark-and-sweep)
• Bounded cache + LRU eviction
• Direct I/O + aligned buffers
• Multi-superblock for atomic superblock update

Longer-term:

• Raise node size to 64–256 KB once journal lands (needs COW write-amp benchmarks)