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).

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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:

  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.

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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.

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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.

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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!