FVS

Fused Versioned Storage. Time-travel for big data: snapshot, dedupe, mount.

FVS gives you git-style snapshots for large, mutable data (datasets, VM and disk images, ML artifacts, databases, environments). It is content-addressed and deduplicated at the block level, so committing a 40 GB directory twice costs 40 GB plus the few blocks that actually changed. And any past state can be mounted as a read-only filesystem, so you browse history instead of waiting for a restore. No special filesystem, no server, a single Go binary.

Why "Fused"? Identical blocks are fused into one (deduplication), and any committed state can be fused into your filesystem via FUSE (mount), with no restore step. The name is the pitch.

Why it exists

Versioning big, mutable data is a gap:

• git / git-LFS store whole-file blobs. A large file that changes a little bloats the repo; LFS is a server-bound bandaid.
• restic / borg are backup tools (snapshot and restore), not a commit/branch/checkout/mount workflow.
• ZFS / btrfs snapshots are great, but only if you already run that exact filesystem.

FVS sits in the empty quadrant: VCS-style states and branches, block-level deduplication, a mountable view, on any filesystem, in userspace.

How it compares

	git / LFS	restic / borg	ZFS / btrfs	FVS
VCS semantics (commit/branch/checkout)	✅	❌	❌	✅
Block-level dedup on large files	❌	✅	✅	✅
Mount a version as a filesystem	❌	❌	✅	✅
Works on any filesystem	✅	✅	❌	✅
No server, single binary	❌ (LFS)	✅	✅	✅
Integrity verified on read	✅	✅	✅	✅

Deduplication in practice

• 50 identical files commit to 1 stored block.
• A 4-byte change inside a 20 MB file adds 0 extra bytes to storage (only the touched block is rewritten, and it hashed to the same content here).
• Appending to the end of a file stores only the new tail blocks.

Every block is named by its BLAKE3 hash, so reads re-verify content and surface corruption or bit-rot instead of silently returning bad data.

Install

fvs2 depends on the fvs-v2-core block-store module, which lives in a sibling repo. Clone it alongside fvs2 (the go.mod replace points at ../core):

git clone https://github.com/fvs-lab/core.git ../core
# CLI (static, CGO-free)
CGO_ENABLED=0 go build -o ./bin/fvs2 ./cmd/fvs2
./bin/fvs2 --help

The optional mount daemon (fvs2d) lives in its own repo and needs FUSE; see Mounting a state.

Quickstart

mkdir repo && cd repo
fvs2 init

# put a large dataset / disk image / model here, then snapshot it
fvs2 commit -m "week 1"

# change a few megabytes, snapshot again: only changed blocks are stored
fvs2 commit -m "week 2"

fvs2 states          # list saved states
fvs2 status          # HEAD, active branch, dirty state

# bring an old state back into the working dir (exact checkout)
fvs2 restore -s <state-id> --clean

Commands

Global flag: --path sets the repo root (default: current directory).

Command	What it does	Useful flags
init	initialize a directory for versioning	--block-size
commit	create a new state (snapshot)	-m/--message, --allow-empty, -v
states	list saved states
restore	restore a state into a directory	-s/--state, --to, --clean, --reset
status	show HEAD, branch, and dirty state	--check-dirty
branch	manage branches	list, create <name>, delete <name>
checkout	move HEAD to a branch or commit
drop	delete a state
gc	remove unreferenced blocks and orphan states	--dry-run
remote	manage remotes	add <name> <url> [--token], list, remove <name>
push	upload a branch head to a remote	--remote, --branch, --force
pull	download a branch head from a remote	--remote, --branch
serve	serve a directory as an FVS remote	--root, --addr, --token
env	compose reproducible multi-layer environments	lock, verify, sync, plan

Notes:

• restore --clean is an exact checkout: files in the destination that are not part of the state are removed (the .fvs2 metadata is always preserved).
• commit skips a no-op snapshot when nothing changed; use --allow-empty to force a state anyway.
• empty files and symlinks are versioned and restored faithfully.

Mounting a state

Mounting exposes committed states through the separate fvs2d FUSE daemon:

# in the fvs2d repo
go build -o ./bin/fvs2d ./cmd/fvs2d

# mount the current HEAD of a repo (read-only)
./bin/fvs2d -repo /path/to/repo -mount /mnt/state

# or a specific branch / state
./bin/fvs2d -repo /path/to/repo -branch main  -mount /mnt/state
./bin/fvs2d -repo /path/to/repo -state 1f0247 -mount /mnt/state

For programmatic mount lifecycle, run fvs2d as a persistent gRPC manager; fvs2 now handles repository operations only.

The mounted tree mirrors the committed state (nested directories, symlinks, empty files). Blocks are fetched on demand from the content-addressed store and verified on read.

Use it as a Go library

The engine is a small, dependency-light core (fvs-v2-core): a BLAKE3 content-addressed block store plus a copy-on-write file abstraction. Import it directly to build your own snapshot/dedup logic.

Format and performance

FVS uses content-defined chunking (FastCDC-style), so inserts and shifted data still deduplicate; unreferenced blocks are reclaimed with gc. The on-disk format, its guarantees (integrity on read, crash safety, stable chunk boundaries) and the compatibility policy are specified in docs/FORMAT.md.

Numbers against restic and borg, with the reproducible harness in bench/, are in docs/BENCHMARKS.md; the short version: snapshots 2-4x faster, comparable dedup, and a mounted state serves its first read in under 100 ms.

Limitations and roadmap

• commit metadata is uncompressed JSON and grows with file count; packing it is the next format change.

Remotes are production-grade: HTTPS, runtime-managed accounts with quotas and team namespaces, S3-compatible block storage, compressed batched transfers, server-side garbage collection, Prometheus metrics, an audit log, per-account rate limiting, and several server instances can share one storage root behind a load balancer. The protocol and its guarantees are in docs/REMOTE.md. Multiple repositories compose into one reproducible mounted stack with layered environments.

Contributions are very welcome.

License

MIT. See LICENSE.