# Duplicacy

## What Is Duplicacy?

[Duplicacy](https://duplicacy.com/) is a backup tool that combines lock-free deduplication with broad cloud storage support. Its defining feature is **cross-computer deduplication** — multiple machines backing up to the same storage can share dedup data, dramatically reducing total storage needs. Duplicacy comes in two editions: a free CLI and a paid web UI ($20 one-time for personal use).

## Prerequisites

- A Linux server (Ubuntu 22.04+ recommended)
- Docker and Docker Compose installed ([guide](/foundations/docker-compose-basics))
- 512 MB of free RAM (minimum)
- A backup destination (local disk, NAS, S3, B2, Wasabi, etc.)

## Docker Compose Configuration

Duplicacy's web UI edition has a Docker image maintained by the community. Create a `docker-compose.yml` file:

```yaml
services:
  duplicacy:
    image: saspus/duplicacy-web:v1.8.3
    container_name: duplicacy
    restart: unless-stopped
    hostname: duplicacy-server
    ports:
      - "3875:3875"    # Web UI
    volumes:
      - duplicacy-config:/config        # Duplicacy configuration and logs
      - duplicacy-cache:/cache          # Dedup cache (speeds up backups)
      - /path/to/backup-source:/data:ro # CHANGE: directories to back up
      - /path/to/local-storage:/storage # CHANGE: local backup destination (optional)
    environment:
      TZ: UTC
      USR_ID: 1000         # Host user ID for file permissions
      GRP_ID: 1000         # Host group ID for file permissions
    networks:
      - backup

networks:
  backup:
    driver: bridge

volumes:
  duplicacy-config:
  duplicacy-cache:
```

**Important:** Replace `/path/to/backup-source` with the directory you want to back up, and `/path/to/local-storage` with your local backup destination. For cloud-only backup, you can remove the `/storage` volume.

Start the server:

```bash
docker compose up -d
```

## Initial Setup

1. Open `http://your-server-ip:3875` in your browser
2. Create an admin password on first access
3. Add a new backup:
   - **Repository:** Path to your data inside the container (e.g., `/data`)
   - **Storage:** Choose your backup destination:
     - Local disk: `/storage`
     - S3: `s3://region@bucket-name`
     - Backblaze B2: `b2://bucket-name`
     - SFTP: `sftp://user@host/path`
     - Wasabi: `wasabi://region@bucket-name`
   - **Schedule:** Set backup frequency (hourly, daily, etc.)

| Default Setting | Value |
|----------------|-------|
| Web UI port | 3875 |
| Default auth | Set on first access |
| Config location | /config (container) |
| Cache location | /cache (container) |

## Configuration

### Supported Storage Backends

| Backend | URI Format | Cost |
|---------|-----------|------|
| Local disk | `/path/to/storage` | $0 (your hardware) |
| SFTP | `sftp://user@host/path` | $0 (your server) |
| Amazon S3 | `s3://region@bucket` | ~$23/TB/month |
| Backblaze B2 | `b2://bucket` | $6/TB/month |
| Wasabi | `wasabi://region@bucket` | $7/TB/month |
| Google Cloud Storage | `gcs://bucket` | ~$20/TB/month |
| Azure Blob | `azure://container` | ~$18/TB/month |
| MinIO (S3-compatible) | `minio://endpoint/bucket` | $0 (self-hosted) |

### Cross-Computer Deduplication

Duplicacy's unique feature: multiple machines can share a single storage backend and deduplicate data across all of them. This is particularly valuable for teams or households where multiple computers contain similar files.

To enable: point all machines at the same storage backend with the same repository ID prefix. Duplicacy handles dedup automatically.

### Retention Policies

Configure in the web UI under each backup's schedule:

| Policy | Description |
|--------|------------|
| Keep all | Keep all snapshots (default) |
| Smart pruning | Keep daily for 7d, weekly for 30d, monthly for 365d |
| Custom | Define your own retention periods |

Example CLI pruning:

```bash
duplicacy prune -keep 0:365 -keep 30:30 -keep 7:7 -keep 1:1
```

This keeps: all snapshots from the last day, daily for 7 days, weekly for 30 days, monthly for a year, then deletes older.

## Advanced Configuration (Optional)

### Encryption

Enable encryption when initializing a repository:

```bash
duplicacy init -e my-repo-id sftp://user@host/backup
```

Duplicacy uses AES-256-GCM encryption. The encryption key is derived from your password — **do not lose it**, as there is no recovery mechanism.

### Filters (Include/Exclude)

Create a `.duplicacy/filters` file in your repository root:

```
# Exclude patterns
-node_modules/
-.git/
-*.tmp
-__pycache__/
-.cache/

# Include patterns
+Documents/
+Photos/
```

### CLI Usage

For users who prefer the free CLI edition:

```bash
# Initialize a repository
cd /path/to/backup-source
duplicacy init my-repo sftp://user@host/backup

# Run a backup
duplicacy backup

# List snapshots
duplicacy list

# Restore from snapshot
duplicacy restore -r 42    # Restore revision 42

# Prune old snapshots
duplicacy prune -keep 0:180 -keep 7:30 -keep 1:7
```

## Reverse Proxy

To access Duplicacy's web UI behind SSL, proxy to port 3875. For setup details, see our [Reverse Proxy Setup](/foundations/reverse-proxy-explained) guide.

## Backup

Back up the Duplicacy configuration itself:

```bash
# The config volume contains all settings and credentials
docker compose exec duplicacy tar czf /tmp/config-backup.tar.gz /config
docker compose cp duplicacy:/tmp/config-backup.tar.gz ./
```

For a comprehensive backup strategy, see our [Backup Strategy](/foundations/backup-strategy) guide.

## Troubleshooting

### Web UI not accessible

**Symptom:** Cannot connect to port 3875.
**Fix:** Check container logs: `docker compose logs duplicacy`. Ensure the port mapping is correct and no other service is using 3875.

### Slow initial backup

**Symptom:** First backup takes much longer than expected.
**Fix:** This is normal — the initial backup uploads all data. Subsequent incremental backups are fast. For large datasets, consider running the first backup to a local destination and then migrating the repository.

### Permission errors on backup source

**Symptom:** "Permission denied" when backing up mounted directories.
**Fix:** Ensure `USR_ID` and `GRP_ID` environment variables match the owner of the source files. Check with `ls -la /path/to/backup-source`.

### Cache growing too large

**Symptom:** The cache volume consumes significant disk space.
**Fix:** Duplicacy caches chunk metadata locally for faster dedup. For very large repositories, the cache can grow to several GB. You can safely delete the cache contents — Duplicacy rebuilds it automatically (first backup after clearing will be slower).

## Resource Requirements

| Resource | Requirement |
|----------|------------|
| RAM | 256 MB idle, 512 MB during backup |
| CPU | Low to Medium (dedup is CPU-intensive during initial backup) |
| Disk | Cache: 1-5 GB. Storage: depends on source data. |

## Verdict

Duplicacy fills a specific niche: **cross-computer deduplication with cloud storage support**. If you back up multiple machines to the same storage, Duplicacy's shared dedup can save 30-50% more space than per-machine tools. The paid web UI ($20 one-time) provides the most polished management experience in the self-hosted backup space. However, for single-machine backup, [Restic](/apps/restic) (free, larger community) or [Kopia](/apps/kopia) (free, built-in UI) are better choices.

## Frequently Asked Questions

### Is Duplicacy free?

The CLI edition is free for personal and commercial use. The web UI edition costs $20 (one-time) for personal use or $50 for commercial use. Both editions support the same backup engine and storage backends. The web UI adds scheduling, status monitoring, and a browser-based management interface.

### What is cross-computer deduplication?

Duplicacy's unique feature: when multiple computers back up to the same storage repository, they share deduplicated chunks. If your desktop and laptop both contain the same 5 GB folder, that data is stored only once in the backup destination. This can reduce total storage needs by 30-50% compared to per-machine deduplication tools like Restic or BorgBackup.

### How does Duplicacy compare to Restic?

Both use chunk-based deduplication and support multiple cloud storage backends. Duplicacy's unique advantage is cross-computer deduplication — multiple machines sharing one storage backend deduplicate across all of them. [Restic](/apps/restic) has a larger community, more documentation, and is fully free. For single-machine backup, Restic is the better default choice. For multi-machine environments, Duplicacy's shared dedup can significantly reduce storage costs.

### Can I encrypt my backups?

Yes. Enable encryption when initializing a repository with the `-e` flag. Duplicacy uses AES-256-GCM encryption. The encryption key is derived from your password — if you lose the password, the backup is unrecoverable. Always store the encryption password in a secure location separate from the backup.

### What cloud storage backends does Duplicacy support?

Duplicacy supports Amazon S3, Backblaze B2, Wasabi, Google Cloud Storage, Azure Blob Storage, SFTP, and any S3-compatible service (including self-hosted [MinIO](/apps/minio)). Local disk backup is also supported. Backblaze B2 ($6/TB/month) and Wasabi ($7/TB/month) offer the best cost for cloud-based backup storage.

### How do I restore files from a Duplicacy backup?

Use the web UI's restore function or the CLI: `duplicacy restore -r REVISION_NUMBER`. You can restore specific files or the entire snapshot. List available snapshots with `duplicacy list`. Each snapshot captures the complete state of your repository at that point in time.

## Related

- [BorgBackup vs Duplicacy: Which Backup Tool Wins?](/compare/borgbackup-vs-duplicacy/)
- [Duplicacy vs Kopia: Which Backup Tool to Self-Host?](/compare/duplicacy-vs-kopia/)
- [Duplicacy vs Restic: Which Backup Tool to Self-Host?](/compare/duplicacy-vs-restic/)
- [Duplicati vs Duplicacy: Backup Tools Compared](/compare/duplicati-vs-duplicacy/)
- [Best Self-Hosted Backup Solutions](/best/backup)
- [Restic vs Kopia vs BorgBackup](/compare/restic-vs-kopia-vs-borgbackup)
- [Duplicati vs Restic](/compare/duplicati-vs-restic)
- [Kopia vs Restic](/compare/kopia-vs-restic)
- [Self-Hosted Alternatives to CrashPlan](/replace/crashplan)
- [Self-Hosted Alternatives to Backblaze](/replace/backblaze)
- [Docker Compose Basics](/foundations/docker-compose-basics)
- [Reverse Proxy Setup](/foundations/reverse-proxy-explained)
- [Backup Strategy](/foundations/backup-strategy)