# backup2mdisc
## How It Works

1. **File Collection & Sorting**  
   - The script uses `find` to list all files in your `SOURCE_DIR` with their sizes.  
   - It sorts them in ascending order by size so it can pack smaller files first (you can remove `| sort -n` if you prefer a different method).

2. **Chunk Accumulation**  
   - It iterates over each file, summing up file sizes into a “current chunk.”  
   - If adding a new file would exceed `CHUNK_SIZE` (default 100GB), it **finalizes** the current chunk (creates `.tar.lz4.gpg`) and starts a new one.  

3. **Archive, Compress, Encrypt**  
   - For each chunk, it creates a `.tar.lz4.gpg` file. Specifically:
     1. `tar -cf - -T $TMP_CHUNK_LIST` (archive of the files in that chunk)  
     2. Pipe into `lz4 -c` for fast compression  
     3. Pipe into `gpg --batch -c` (symmetric encrypt with AES256, using your passphrase)  
   - The result is a self-contained file like `chunk_001.tar.lz4.gpg`.

4. **Checksums & Manifest**  
   - It calculates the SHA-256 sum of each chunk archive and appends it to a manifest file along with the list of included files.  
   - That manifest is stored in `$WORK_DIR`.

5. **Optional ISO Creation** (`--create-iso`)  
   - After each chunk is created, the script can build an ISO image containing just that `.tar.lz4.gpg`.  
   - This step uses `genisoimage` (or `mkisofs`). The resulting file is `chunk_001.iso`, etc.  

6. **Optional Burning** (`--burn`)  
   - If you specify `--burn`, the script will pause after creating each chunk/ISO and prompt you to insert a fresh M-Disc.  
   - On **Linux**, it tries `growisofs`.  
   - On **macOS**, it tries `hdiutil` (if creating an ISO).  
   - If it doesn't find these commands, it'll instruct you to burn manually.  

7. **Repeat**  
   - The script loops until all files have been placed into chunk(s).  

---

## Restoring Your Data

- **Disc is self-contained**: If you have disc #4 containing `chunk_004.tar.lz4.gpg`, you can restore it independently of the others.  
- **Decrypt & Extract**:  
  ```bash
  gpg --decrypt chunk_004.tar.lz4.gpg | lz4 -d | tar -xvf -
  ```
  This will prompt for the passphrase you used during backup.

- If one disc is lost, you only lose the files in that chunk; all other chunks remain restorable.

---

## Why lz4?

- **Speed**: `lz4` is extremely fast at both compression and decompression.  
- **Less compression ratio** than xz, but if your priority is speed (and 100GB disc space is enough), `lz4` is a great choice.  
- For maximum compression at the cost of time, you could replace `lz4` with `xz -9`, but expect slower backups and restores.

---

## Tips & Caveats

1. **Large Files**  
   - A single file larger than your chunk size (e.g., 101GB file with a 100GB chunk limit) won't fit. This script doesn't handle that gracefully. You'd need to split such a file (e.g., with `split`) before archiving or use a backup tool that supports partial file splitting.  

2. **Verification**  
   - Always verify your discs after burning. Mount them and compare the chunk's SHA-256 with the manifest to ensure data integrity.  

3. **Incremental or Deduplicated Backups**  
   - For advanced features (incremental, deduplication, partial-chunk checksums), consider specialized backup programs (like Borg, restic, Duplicati). However, they usually produce multi-volume archives that need **all** volumes to restore.  

4. **Cross-Platform**  
   - On FreeBSD or macOS, you might need to tweak the commands for hashing (`sha256sum` vs. `shasum -a 256`) or ISO creation (`mkisofs` vs. `genisoimage`).  
   - For burning, Linux uses `growisofs`, macOS uses `hdiutil`, and FreeBSD may require `cdrecord` or another tool.

---

**Now you can enjoy the best of both worlds**:  
- **Independently decryptable** (and restorable) archives on each M-Disc.  
- Automatic ISO creation and optional disc burning in the same script.  
- Fast compression via lz4.  

This gives you a **self-contained** backup on each disc without chain-dependency across your entire 2TB backup set!