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# ⚡ FastSync
**Fast folder sync, written in Rust.**
Mirror a source folder into a target folder, or share a folder once over the network, with speed, clear previews, and safer overwrite behavior.
[](LICENSE)
[](https://www.rust-lang.org/)
[](https://doc.rust-lang.org/edition-guide/rust-2024/index.html)
[](https://github.com/BLAKE3-team/BLAKE3)
[](https://github.com/ShouChenICU/FastSync)
[简体中文](README.zh-CN.md) · [Extreme Performance](#-extreme-performance) · [Network Sync](#-remote-folder-sync) · [Safety](#-safety-first-by-default) · [Install](#-install) · [CLI](#-cli-cheat-sheet)
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| Rust, metadata-aware comparison, BLAKE3, concurrent workers | One-shot share/connect with a 6-digit code | Avoids leaving corrupted partial files after interruption |
## ✨ Why FastSync?
FastSync is built for large folders and short-lived directory handoffs where speed matters, but silent mistakes are unacceptable.
- **Written in Rust**: fast native execution, predictable resource use, and a small deployment story.
- **Fast by design**: metadata-aware comparison, BLAKE3, and concurrent workers.
- **Network sync built in**: share or receive a folder once with a simple pairing code.
- **Safe by default**: no implicit deletion, dry-run support, and temporary-file overwrite writes.
- **Clear after every run**: readable summaries for humans, JSON for scripts.
```mermaid
flowchart LR
A["Source folder"] --> B["Scan"]
C["Target folder"] --> B
B --> D["Compare"]
D --> E["Copy / update"]
D --> F["Optional delete"]
E --> G["Readable summary"]
F --> G
```
## 🏎️ Extreme Performance
Directory sync is a mix of filesystem latency, metadata checks, hashing, and copying. FastSync keeps those stages explicit and controlled.
| Rust implementation | Native binary performance with predictable memory and CPU behavior. |
| Metadata-aware comparison | Uses file size and modified time where they are valid content signals, while metadata synchronization stays separately configurable. |
| BLAKE3 hashing | Uses a very fast modern hash for strong content comparison when needed. |
| Bounded worker queue | Keeps copying concurrent without letting memory usage grow without control. |
| Direct new-file copy | Files missing from the target are copied directly, avoiding unnecessary temporary rename overhead. |
> [!NOTE]
> Fast comparison is the default. Use `--strict` when same-metadata files should still be confirmed with BLAKE3.
## 🚀 Quick Start
Preview the sync:
```bash
fastsync -n ./source ./target
```
Run it for real:
```bash
fastsync ./source ./target
```
Mirror and remove stale target files:
```bash
fastsync -n -d ./source ./target
fastsync -d ./source ./target
```
> [!CAUTION]
> `--delete` removes files from the target when they do not exist in the source. Preview with `-n -d` before the first real deletion run.
## 📦 Install
FastSync uses the Rust 2024 edition and requires Rust 1.85 or newer. With `rustup`, use the stable toolchain:
```bash
rustup default stable
rustup component add rust-src
```
### Install from crates.io
```bash
cargo install fastsync
fastsync --help
```
### Build from source
```bash
git clone https://github.com/ShouChenICU/FastSync.git
cd FastSync
cargo build --release
./target/release/fastsync --help
```
### Install from Git
```bash
cargo install --git https://github.com/ShouChenICU/FastSync
```
## 🌐 Language
FastSync supports English and Simplified Chinese. It detects common system locales automatically, and you can override the language when needed:
```bash
fastsync --lang zh-CN --help
FASTSYNC_LANG=zh-CN fastsync --help
```
## 🧭 Common Workflows
| Preview a sync | `fastsync -n ./source ./target` |
| Sync one folder into another | `fastsync ./source ./target` |
| Sync and delete stale target files | `fastsync -d ./source ./target` |
| Use strict comparison | `fastsync --strict ./source ./target` |
| Limit worker threads | `fastsync -t 4 ./source ./target` |
| Output JSON for scripts | `fastsync -o json ./source ./target` |
| Share a folder once | `fastsync s ./source` |
| Receive a shared folder | `fastsync c host ./target -c 123456` |
<details>
<summary><strong>Example: safe backup mirror</strong></summary>
```bash
# First run: inspect what would happen.
fastsync -n -d ~/Photos /mnt/backup/Photos
# Second run: apply the same operation.
fastsync -d ~/Photos /mnt/backup/Photos
```
</details>
<details>
<summary><strong>Example: fast cache mirror</strong></summary>
```bash
fastsync ./target/release ./cache/release
```
The default fast mode trusts matching metadata, then hashes only when same-size files have differing modified times or supported permissions.
</details>
## 🌐 Remote Folder Sync
Use this for a temporary handoff: send a folder to someone, or let them upload one to you. The person sharing the folder starts `share`, reads out the one-time code, and the other side runs `connect`.
> [!IMPORTANT]
> This is one-way sync. Choose download or upload for each session; FastSync does not merge changes from both sides.
Send a folder to someone:
```bash
fastsync s ./photos
fastsync c server.example.com ./photos -c 123456
```
Let someone upload a folder to you:
```bash
fastsync s ./inbox -r
fastsync c server.example.com ./project -u -c 123456
```
What happens by default:
| share sends files | `fastsync s ./photos` only lets the other side download. |
| one-time code | FastSync prints a code when sharing starts. |
| one successful use | The sharing side exits after one completed sync. |
| no server deletion | Upload clients cannot delete your files unless you explicitly allow it. |
You can omit `--code`; FastSync will prompt for it.
Common shortcuts:
| `share` / `connect` | `s` / `c` |
| `--code 123456` | `-c 123456` |
| `--mode receive` | `-r` or `-m r` |
| `--direction push` | `-u` |
| `--delete` | `-d` |
| `--strict` | no short form |
| `--allow-delete` | `-a` |
| `--preserve-permissions` | `-p` or `--perms` |
Deleting extra files is always opt-in and only affects the side receiving files:
| download | Extra files in your local folder | None |
| upload | Extra files in the shared folder | Sharing side must allow deletion |
```bash
fastsync c server.example.com ./photos -d -c 123456
fastsync s ./inbox -r -a
fastsync c server.example.com ./project -u -d -c 123456
```
By default, received files keep their modification times. Permission bits are copied only when requested:
| `--strict` | Hash same-size local files even when metadata matches before deciding what to transfer. |
| `--no-preserve-times` | Do not preserve source modification times on received files and directories. |
| `--preserve-permissions` | Preserve source permission bits on received files and directories. Disabled by default. |
Without `--strict`, network sync defaults to fast comparison: matching metadata is trusted, and BLAKE3 is used only for same-size files whose metadata differs.
For auditing, the sharing side logs who connected, whether the session downloaded or uploaded, delete/metadata choices, pairing failures, file count, byte count, deleted count, and elapsed time. Use `--log-level debug` for more detail.
Technical note: one-shot network sync uses QUIC with a temporary self-signed certificate, verifies received files with BLAKE3, and writes through temporary files before replacement. Use it for short-lived sessions where both sides can confirm the address and code.
## 🛡️ Safety First By Default
| One-way sync | The source is the authority; the target follows it. |
| No implicit deletion | Target-only files are preserved unless `--delete` is used. |
| Fast comparison | Existing files trust matching metadata by default, and use BLAKE3 only for same-size files whose metadata differs. |
| Temporary-file overwrite | Existing targets are written to a temporary filename first, then renamed into place, reducing the chance of leaving a partial file after interruption. |
| Direct new-file copy | Missing target files are copied directly, without unnecessary rename overhead. |
| Dry-run support | You can inspect the plan before changing anything. |
## 🔍 Choose A Comparison Mode
| `fast` | If metadata matches, treats the file as unchanged. If metadata differs, size differences are changed immediately; same-size files are checked with BLAKE3. This is the default. | You want good speed while still hashing ambiguous same-size changes. |
| `strict` | If sizes match, checks content with BLAKE3 even when metadata also matches. | You want content confirmation for every existing same-size file. |
`--strict` is a shortcut for `--compare strict`.
> [!IMPORTANT]
> Fast mode can miss content changes when size, modified time, and supported permissions stay the same. Use `strict` for important data that needs content confirmation even when metadata matches.
Same-name file metadata synchronization is separate from content comparison and is enabled by default. Use `--no-sync-metadata` to skip standalone metadata updates, or `--preserve-times false` and/or `--preserve-permissions false` to narrow which metadata is preserved.
## ✅ Verification
Post-copy verification is controlled by `--verify`:
| `none` | Do not verify after copying. |
| `changed` | Verify overwritten files. This is the default. |
| `all` | Verify all regular source files after sync. |
The summary reports BLAKE3 content checks in two separate counters: comparison-time checks used by `fast` or `strict`, and post-copy verifications controlled by `--verify`.
New files that do not exist in the target are copied directly and are not counted as post-copy BLAKE3 verifications.
## 🧾 CLI Cheat Sheet
| `-n`, `--dry-run` | Preview only; do not modify the target. |
| `-d`, `--delete` | Delete target entries that no longer exist in the source. |
| `--strict` | Use strict BLAKE3 confirmation for same-size existing files. |
| `-c`, `--compare <fast\|strict>` | Select the comparison strategy. |
| `--no-sync-metadata` | Do not update metadata for same-name files whose content already matches. |
| `--preserve-times <auto\|true\|false>` | Control timestamp synchronization. |
| `--preserve-permissions <auto\|true\|false>` | Control permission synchronization. |
| `--verify <none\|changed\|all>` | Select post-copy verification. |
| `-t`, `--threads <N\|auto>` | Set the worker count. |
| `-q`, `--queue-size <N>` | Set the bounded task queue size. |
| `--no-atomic-write` | Disable temporary-file overwrite writes. |
| `-o`, `--output <text\|json>` | Select summary format. |
| `-l`, `--log-level <level>` | Set log verbosity. |
| `--lang <en\|zh-CN>` | Select interface language. Also accepts common locale aliases. |
Network one-shot commands:
| `fastsync share <DIRECTORY>` | Start a temporary server. Defaults to `--mode send`. |
| `fastsync connect <ENDPOINT> <DIRECTORY>` | Connect to a temporary server. Defaults to `--direction pull`. |
| `fastsync s <DIRECTORY>` | Short form of `fastsync share`. |
| `fastsync c <ENDPOINT> <DIRECTORY>` | Short form of `fastsync connect`. |
| `fastsync c <ENDPOINT> <DIRECTORY> --strict` | Use strict comparison before requesting files. |
| `fastsync share --help` | Show all server options. |
| `fastsync connect --help` | Show all client options. |
Print the full help pages:
```bash
fastsync --help
fastsync share --help
fastsync connect --help
```
Running `fastsync` without arguments also prints help.
## 🧪 Development
This crate sets `edition = "2024"` in `Cargo.toml`. That is the Rust edition name, not the current calendar year; Rust editions are opt-in language compatibility milestones, and the 2024 edition remains current even when building in 2026.
```bash
cargo fmt --check
cargo test
cargo clippy --all-targets --all-features -- -D warnings
```
Maintainers and coding agents should read [AGENTS.md](AGENTS.md).
## ❓ FAQ
<details>
<summary><strong>Is FastSync bidirectional?</strong></summary>
No. FastSync is intentionally one-way: source to target.
</details>
<details>
<summary><strong>Will FastSync delete files by default?</strong></summary>
No. Deletion only happens when `--delete` or `-d` is provided.
</details>
<details>
<summary><strong>Should I use <code>--strict</code>?</strong></summary>
Use it for important personal or production data where matching metadata is not enough confidence. For generated files, caches, and build outputs, the default `fast` mode is usually the better tradeoff.
</details>
## 📄 License
FastSync is open source under the [MIT License](LICENSE).
Author: [ShouChen](https://github.com/ShouChenICU)
Repository: [https://github.com/ShouChenICU/FastSync](https://github.com/ShouChenICU/FastSync)