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# ffsync
**Fast chunk-level file sync for big files and fast links.**
ffsync transfers the difference between two files, not the files. It is rsync's
behavior — minimal bytes on the wire, resumable, verified — built on a modern
engine: parallel content-defined chunking, BLAKE3 hashing, zstd compression,
and a hand-rolled, dependency-light wire protocol. No daemon, no config, two
commands.
[](LICENSE-MIT)
[](https://www.rust-lang.org)
[](CHANGELOG.md)
[](https://github.com/bennischober/ffsync/actions/workflows/ci.yml)
</div>
> **Status: v0.3.1.** Single-file **and directory-tree** sync between two
> peers over TLS work end-to-end, including resume after mid-transfer
> disconnects. Trees sync by metadata triage (D17): only changed files get
> conversations, synced files adopt the sender's mtime, and a run with
> nothing changed reads metadata only. Deletions of receiver-only files
> apply after every transfer succeeded, opt-out with `--no-delete`. Not
> implemented yet: exclude patterns, parallel streams (QUIC). See
> [roadmap](#roadmap) below.
## Why
rsync is battle-tested, but its per-file scan is single-threaded: the sender
computes a rolling checksum over every byte on one core. On large files that
means a long "cold start" of disk-grinding, network-silent hashing before the
first byte moves — and rsync pays that cost again on every run.
Measured during development on a 72.7 GiB file (SATA SSD, ~550 MB/s):
| rsync 3.1.3 | ~15 min | ~85 MB/s (single core) |
| ffsync scan | ~242 s | ~320 MB/s (disk-bound, all cores) |
That is a **~3.8x faster cold start**, and the gap widens on faster hardware:
BLAKE3 is SIMD-parallel while rsync's checksum work is not.
Memory stays flat while files grow: chunk data is never copied — the file is
memory-mapped and hashed in place, channels carry `(offset, length)` with
backpressure, and the receiver writes positionally from a reused buffer. A
1 GiB sync peaks at ~3–7 MiB of heap (measured,
[`evaluation/memory.md`](evaluation/memory.md)); all sync state is O(chunks),
~130 B per chunk — a 70 GiB movie syncs on roughly 35 MiB of RAM. Trees add
~90 B per path; 10× the files does not move the needle.
The honest niche: sync time ≈ max(scan on both peers, transfer). On slow links
transfer dominates and rsync is fine. On **fast links (LAN, 1–10 GbE) with big
files and repeated syncs**, the scan dominates — that is the cost ffsync
attacks. The full reasoning, including where ffsync is *worse* than rsync, is
in [`concept/Positioning.md`](concept/Positioning.md).
## How it works
```mermaid
flowchart LR
subgraph sender["sender"]
A[source file] --> B[parallel scan<br/>FastCDC + BLAKE3<br/>mmap, one chunker/core]
end
subgraph receiver["receiver"]
C[old file] --> D[parallel scan]
D --> E[manifest<br/>hash, offset, length]
end
B -->|chunk hashes| F[diff against manifest]
E --> F
F -->|plan: local vs remote| G[zstd delta transfer]
G --> H[out-of-place reconstruction<br/>pre-allocated temp, positional writes]
H --> I[chunk-root verification<br/>atomic rename]
```
1. **Scan** — the file is memory-mapped and chunked with FastCDC
(content-defined boundaries, 64 KiB / 256 KiB / 1 MiB by default) on one
thread per core; every chunk is hashed with BLAKE3 on worker threads.
Chunks travel through bounded channels as `(offset, length)` — content is
never copied.
2. **Exchange** — the receiver streams its manifest (chunk hashes); the sender
diffs and produces a plan: chunks the receiver already has (by exact hash
match) are named, missing chunks are scheduled for transfer.
3. **Transfer** — missing chunks are zstd-compressed and streamed; the
receiver executes the plan before the data arrives, reading local chunks
from its own file (locals-first write order — this is what makes resume
work).
4. **Verify and commit** — the reconstructed file is re-scanned, its
chunk-tree root (a domain-separated BLAKE3 over the ordered chunk hashes)
must equal the sender's, and only then does a temp file atomically replace
the target. A wrong byte anywhere fails loudly, never silently.
5. **Warm start (after the first sync)** — each side persists a per-file
chunk index (`<file>.ffsync-index`) at commit time, holding the chunk list
and root it just verified. On the next sync, a peer whose file still
matches the index (size + mtime, compared at nanosecond granularity)
skips the scan entirely and serves its manifest/plan from the index —
repeated syncs of unchanged data become stat-only operations. rsync
rescans everything from scratch on every run; this is the memory rsync
does not have. Corrupt indexes are rejected at load (the decoder
re-derives the chunk root); a stale-but-validating index fails loudly and
is dropped, so a retry cold-scans — wrong content can cost an error,
never a silent mismatch.
6. **Tree mode (v0.3)** — a directory path switches from one file to a
whole tree: both peers exchange manifests of `(relative path, kind, size,
mtime)` (metadata only, no contents), triage locally and identically,
and then run the per-file machinery above — sequentially, over the same
connection — for exactly the files that need it. Matching files are
skipped without a byte of I/O; synced files adopt the sender's mtime so
the next run skips them again. Files only the receiver has are deleted
last, after every transfer succeeded (`--no-delete` keeps them).
The wire protocol is specified in
[`concept/Protocol v0.1.md`](concept/Protocol%20v0.1.md): 14 frame types,
length-prefixed, no protobuf.
## Built on
| [FastCDC](https://github.com/nlfiedler/fastcdc-rs) (`fastcdc` v2020) | Content-defined chunking | Cut points survive insertions/deletions; chunks stay self-identifying across file versions |
| [BLAKE3](https://github.com/BLAKE3-team/BLAKE3) | Chunk + tree hashing | Extremely fast (SIMD, multi-threaded), one primitive for chunk IDs, verification, and the file identity |
| [zstd](https://github.com/facebook/zstd) | Chunk compression | Measured best ratio/speed trade-off for chunk-sized payloads |
| [memmap2](https://github.com/RazrFalcon/memmap2-rs) | File I/O | Zero-copy slicing for hash workers; no read() syscalls per chunk |
| [crossbeam](https://github.com/crossbeam-rs/crossbeam) | Bounded channels | Backpressure between chunkers and hash workers on native threads; no async runtime in the data plane |
| [rustls](https://github.com/rustls/rustls) + [ring](https://github.com/briansmith/ring) | Transport security (D16) | TLS 1.3 as a thin `Read + Write` wrapper around TCP; the audited, mainstream Rust choice — no hand-rolled channel crypto |
| [rcgen](https://github.com/rustls/rcgen) | Identity certs | Generates the self-signed peer certificate on first run |
| rsync (comparison only) | Baseline | The tool this project is measured against, and its delta algorithm is respected, not wrapped |
Design decisions and their alternatives are recorded in
[`concept/Design Decisions.md`](concept/Design%20Decisions.md) (D1-D17).
## Using it
Install (three routes, fastest first for most users):
```bash
# 1. Download a prebuilt executable from the releases page (Windows zip,
# Linux tar.gz; SHA-256 sidecars included) - no toolchain needed:
# https://github.com/bennischober/ffsync/releases/latest
# 2. From crates.io (needs a Rust toolchain):
cargo install ffsync
# 3. From the repository (tracks main):
cargo install --git https://github.com/bennischober/ffsync
```
Binaries do not auto-update; new releases are announced on the releases
page. Verify a download against the attached `.sha256` sidecar
(`sha256sum -c ffsync-*.sha256` / `Get-FileHash` on Windows).
Two terminals on the same machine (or LAN - replace `127.0.0.1` with the
receiver's address):
```bash
# Receiver: listen and converge ./big-file.bin from its current state
ffsync receive ./big-file.bin 7788
# Sender: push the (newer) file to the receiver
ffsync send ./big-file.bin 127.0.0.1:7788
```
**Directory trees** (v0.3) — pass a directory instead of a file; the
*contents* of the directory sync into the destination directory (`dir` and
`dir/` are the same thing, no rsync trailing-slash semantics):
```bash
# Receiver: converge ./photos toward the sender's tree
ffsync receive ./photos 7788
# Sender: push the whole tree
ffsync send ./photos 127.0.0.1:7788
```
Tree mode syncs files whose size or modification time differs, creates
missing directories (including empty ones), and **deletes** files that
exist only on the receiver — the sender's tree is authoritative, like
`rsync --delete`. Pass `--no-delete` on receive to keep receiver-only
files; deletions apply only after every transfer succeeded (a failed sync
never destroys data). Because synced files carry the sender's mtime, a
second run with nothing changed reads metadata only — no hashing, no
transfer, a handful of frames.
**Transport security is on by default** (D16): both peers generate a
self-signed certificate on first run and authenticate each other by
fingerprint. On first contact each side prints the peer's fingerprint and
asks `trust this peer? [y/N]`; compare it out-of-band (in person, chat,
whatever you would use to exchange a key), confirm once, and every later
sync verifies silently. A fingerprint mismatch refuses loudly and syncs
nothing. For scripts, pass `--trust <fingerprint>` instead of the prompt;
`ffsync trust <fingerprint>` / `ffsync untrust <fingerprint>` /
`ffsync fingerprints` manage the pins (`pins.conf` in the ffsync state
directory - no usernames, no passwords, no secrets on disk). Use
`--plaintext` for trusted LANs or inside an SSH tunnel.
Flags: `--checksum` disables the mtime/size fast path *and* the warm-start
index (rsync `-c` semantics - always compare chunk roots; in tree mode it
applies to every file); receiver-side (single-file) `--from <old-file>`
reads the local chunks from a different file than the target. If the
connection drops mid-transfer, the receiver keeps its partial state;
rerunning the same two commands **resumes** - for a single file, the
receiver re-scans its temp file, unions it with the old file, and only the
genuinely missing chunks cross the wire; for a tree, files already synced
before the drop match in metadata and are skipped, so a rerun continues
where the session died (plus the interrupted file's per-file resume).
After a successful sync, both peers keep `<file>.ffsync-index` sidecars;
the next run of an unchanged file shows
`warm loaded N chunks from index, scan skipped` instead of a hashing phase.
Deleting the index file (or the file itself, whose orphaned index is cleaned
up automatically) always falls back to a full cold scan.
Progress is single-line: `hashing → diffing → sending/receiving → verifying`,
with bytes, throughput, and elapsed time per phase. A scan-bound sync says so
instead of being silently disk-busy/network-idle — that silence in rsync is
why this project exists.
## Benchmark
32 MiB fixtures, seeded pseudo-random data, release build, all reconstructions
hash-verified (`cargo run --release --bin bench -- --quick --seed 42 --rsync`):
```
scenario src scan tgt scan missing delta % dedup recon e2e 1x verified
identical 27.5 ms 25.5 ms 0 B 0.00% inf 47.0 ms 27.5 ms yes
modified 29.3 ms 26.8 ms 1.53 MiB 4.79% 20.9x 47.3 ms 42.1 ms yes
appended 27.6 ms 27.4 ms 3.24 MiB 9.21% 10.9x 51.0 ms 54.8 ms yes
fresh 27.7 ms 25.1 ms 32.00 MiB 100.00% 1.0x 39.3 ms 296.2 ms yes
```
A 4.79% modification sends 4.79% of the bytes; identical files cost zero
transfer bytes and one parallel scan. On the same modified scenario, rsync's
delta algorithm (`--no-whole-file` via WSL, same fixtures) sent **667 KiB** —
its fine-grained fixed blocks win on delta ratio for small localized edits.
ffsync's 256 KiB average chunks trade some of that for scan parallelism; the
chunk-size sweep that picked the defaults is in
[`evaluation/rsync.md`](evaluation/rsync.md).
Caveats: 32 MiB is page-cache resident and the fixtures are random data (worst
case for compression, best case for CDC boundary stability). Treat the numbers
as structural, not as your-hardware numbers; the harness reproduces them with
`--size-mib 1024` on real-sized files.
**Tree sessions** (`--tree`, D17): a 40-file / 135 MiB fixture across 6 dirs
run through three simulated sessions — everything transfers when the receiver
is empty, a diverged session moves only the changed bytes, and the warm
unchanged session is stat-only:
```
session 1 (receiver empty) scan 0.9 ms 40 conversations 135.0 MiB transferred
session 2 (diverged, cold) scan 1.5 ms 15 conversations 17.44 MiB transferred (54.5% of touched files)
session 3 (unchanged, warm) scan 1.5 ms 0 conversations 0 B transferred
```
The unchanged session skips hashing entirely (mtime preservation at commit
makes the metadata fast path hold tree-wide); the diverged session deletes
receiver-only files only after every conversation succeeded. Measured details
in [`evaluation/chunking.md`](evaluation/chunking.md).
## Building and benchmarking
```bash
cargo build --release
cargo test
cargo run --release --bin bench -- --quick # fast run (32 MiB)
cargo run --release --bin bench -- --size-mib 1024 # 1 GiB, stresses the pipeline
cargo run --release --bin bench -- --tree --seed 42 # tree sessions (D17)
cargo run --release --bin bench -- --quick --rsync # also time rsync (native or WSL)
cargo audit # dependency advisory check
```
Release executables are built by the tag-driven release workflow: Windows
(MSVC, static CRT — runs without the VC++ redistributable) and Linux (musl,
fully static), each with a SHA-256 sidecar attached to the GitHub release.
Requires a stable Rust toolchain (developed on 1.99, edition 2024). Windows is
the primary development platform; the I/O layer is `#[cfg]`-split for Unix.
## Project layout
```
src/
chunking/ the pipeline: mmap -> FastCDC chunkers -> BLAKE3 hash workers
reconstruction/ out-of-place rebuild: temp file, positional writes, verify, rename
protocol/ wire protocol: framing, messages, state machines, tree sessions
tree.rs tree scan, path validation, triage (D17)
index.rs persistent chunk index (warm starts): codec, validate, atomic write
transport.rs TLS + fingerprint pinning (D16)
main.rs the CLI
bin/bench.rs the benchmark harness
concept/ design docs (positioning, decisions, protocol spec, research)
evaluation/ measured results (chunking, rsync comparison)
```
## Design docs
The reasoning is the product; it is all recorded:
- [`concept/Positioning.md`](concept/Positioning.md) — why this exists, the
niche, why "build around rsync" does not work
- [`concept/Design Decisions.md`](concept/Design%20Decisions.md) — D1–D17 with
alternatives and rejected options
- [`concept/Protocol v0.1.md`](concept/Protocol%20v0.1.md) — the wire protocol
(tracks all versions; currently v3), frame by frame
- [`concept/Deep Research Digest.md`](concept/Deep%20Research%20Digest.md) —
what the two deep-research runs got right, and what was rejected
- [`evaluation/chunking.md`](evaluation/chunking.md) and
[`evaluation/rsync.md`](evaluation/rsync.md) — the measurements behind the
defaults
## Roadmap
- **Done (0.2.0) - transport security (D16):** TLS 1.3 by default,
mutual authentication by self-signed-cert fingerprint pinning (TOFU with
an out-of-band compare on first contact); `--plaintext` keeps LAN/SSH
tunnels. QUIC is deferred until measurement shows parallel streams
worthwhile (D13 amendment).
- **Done (0.2.0) - persistent chunk index (warm restarts, D15):** chunks
are content-addressed, so a persisted index turns repeated syncs into
stat-only operations. This is the core product insight; rsync rescans
everything from scratch every run.
- **Done (0.3.0) - directory-tree sync (D17):** metadata triage over a
tree, sequential per-file conversations over one connection, mtime
preservation, deferred deletes with `--no-delete`, empty files/dirs
first-class.
- **Next - excludes/pattern matching** (`node_modules`, `.git`): the most
requested missing piece for real trees; deliberately not shipped in 0.3
(a wrong exclude silently syncs the wrong set — worth doing properly).
- **Later - parallel per-file streams / QUIC transport** (D13): connection
migration for the "friend over the internet" case; plain TCP and
TLS-over-TCP stay available.
- **Later - SeqCDC and other chunkers** as they mature; the `Chunker` trait
makes swapping cheap (registered as F2).
## License
Dual-licensed under either of
- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE))
- MIT license ([LICENSE-MIT](LICENSE-MIT))
at your option.