When dealing with large files (like your 1GB web backup), Dropbox employs a smart chunking algorithm. Instead of treating files as monolithic blocks, it breaks them into smaller chunks (typically 4MB each). This allows for efficient delta synchronization.

// Conceptual representation of chunk hashing
function generateChunkHashes(file) {
  const CHUNK_SIZE = 4 * 1024 * 1024; // 4MB
  const chunks = [];
  let offset = 0;
  
  while (offset < file.size) {
    const chunk = file.slice(offset, offset + CHUNK_SIZE);
    const hash = sha256(chunk); // Dropbox uses custom hashing
    chunks.push({ offset, hash });
    offset += CHUNK_SIZE;
  }
  return chunks;
}

For your web backup scenario, Dropbox will:

• Compare existing chunk hashes with new file versions
• Only upload modified chunks (not the entire 1GB file)
• Maintain previous versions without duplicating unchanged chunks

A real-world example with your web files:

You can check this behavior using Dropbox's API:

// Example using Dropbox API v2
const dbx = new Dropbox.Dropbox({ accessToken: 'YOUR_TOKEN' });

dbx.filesGetMetadata({ path: '/web_backup.zip' })
  .then(response => {
    console.log('Content hash:', response.content_hash);
    console.log('Current version:', response.rev);
  });

Important notes about version storage in free accounts:

• Deleted files: Count against quota until version history expires (30 days)
• Modified files: Only store delta changes, not full copies
• Version retention: Free accounts keep previous versions for 30 days

To minimize bandwidth usage:

#!/bin/bash
# Example pre-upload script to compress only modified files
find /path/to/web/files -mtime -1 -exec zip -u backup.zip {} +

This approach ensures Dropbox only processes actually modified content rather than re-syncing the entire archive.

When working with large files (like your 1GB web backup), Dropbox uses a clever technique called delta synchronization. Instead of re-uploading the entire file each time, it only transfers the changed portions. Here's how it works at a technical level:

// Simplified conceptual representation
function deltaSync(oldFile, newFile) {
    const chunkSize = 4 * 1024 * 1024; // 4MB chunks (Dropbox's actual block size)
    const oldHashes = computeBlockHashes(oldFile, chunkSize);
    const newHashes = computeBlockHashes(newFile, chunkSize);
    
    const diff = compareHashes(oldHashes, newHashes);
    return {
        unchangedBlocks: diff.matchingIndices,
        newBlocks: extractChangedBlocks(newFile, diff.nonMatchingIndices, chunkSize)
    };
}

For version history on free accounts (2GB plan):

• 30-day version history by default
• Each version stores only the delta changes
• Storage counted against your quota only for current version + deltas

If you're building similar functionality or optimizing your workflow:

# Python example using Dropbox API v2
import dropbox

dbx = dropbox.Dropbox('YOUR_ACCESS_TOKEN')

def smart_upload(file_path):
    with open(file_path, 'rb') as f:
        file_size = os.path.getsize(file_path)
        
        # For files over 150MB, Dropbox requires chunked upload
        if file_size <= 150 * 1024 * 1024:
            dbx.files_upload(f.read(), '/' + os.path.basename(file_path))
        else:
            upload_session_start_result = dbx.files_upload_session_start(f.read(4 * 1024 * 1024))
            cursor = dropbox.files.UploadSessionCursor(
                session_id=upload_session_start_result.session_id,
                offset=f.tell()
            )
            while f.tell() < file_size:
                if (file_size - f.tell()) <= 4 * 1024 * 1024:
                    dbx.files_upload_session_finish(
                        f.read(4 * 1024 * 1024),
                        cursor,
                        dropbox.files.CommitInfo(path='/' + os.path.basename(file_path))
                    )
                else:
                    dbx.files_upload_session_append(
                        f.read(4 * 1024 * 1024),
                        cursor.session_id,
                        cursor.offset
                    )
                    cursor.offset = f.tell()

For your specific 1GB web backup scenario:

1. Initial upload: Only happens once (the full 1GB)
2. Subsequent syncs: Typically transfers <1% of file size for minor changes
3. Version storage: Dropbox uses block-level deduplication across versions

You can verify the efficiency with:

# Linux/Mac command to monitor Dropbox network usage
lsof -i | grep dropbox
# Or for Windows:
netstat -b | findstr dropbox

For programmatic monitoring, Dropbox's API provides endpoints to check sync status and bandwidth usage.