While both --checksum and --ignore-times alter rsync's default comparison behavior, they operate at different levels of the file synchronization process:

# Default rsync behavior (without either flag):
rsync -avz /source/ user@remote:/destination/

# With --checksum:
rsync -avz --checksum /source/ user@remote:/destination/

# With --ignore-times:
rsync -avz --ignore-times /source/ user@remote:/destination/

--checksum adds an additional verification layer after size and timestamp comparison. When files appear identical by size and mtime, rsync:

1. Calculates MD5 checksums for both files
2. Compares the checksums byte-by-byte
3. Only skips transfer if checksums match exactly

--ignore-times completely bypasses the timestamp comparison phase, forcing rsync to:

1. Always check file sizes
2. Perform delta-transfer analysis regardless of modification times
3. Transfer only the differing portions if files aren't identical

The CPU and I/O overhead differs significantly:

# Benchmark command template
time rsync [OPTIONS] large_dir/ remote:backup/

--checksum impact:

• +15-30% CPU usage during initial comparison phase
• +5-10% total sync time for large directories
• Significant disk I/O for checksum calculation

--ignore-times impact:

• Minimal CPU overhead compared to default
• Faster than --checksum but slower than default
• No additional disk I/O beyond normal transfer

When to use --checksum:

# Backup systems where timestamps might get reset
rsync --checksum --delete /critical-data/ backup-server:/archive/

# Version control systems with identical timestamps
rsync --checksum --exclude='.git/' repo/ mirror-server:/repositories/

When to use --ignore-times:

# Continuous integration systems with frequent rebuilds
rsync --ignore-times --filter='P *.o' build/ test-machine:/build-artifacts/

# Cloud deployments with inconsistent timestamp precision
rsync --ignore-times --exclude='node_modules/' app/ production:/deploy/

For maximum reliability in critical scenarios:

# Nuclear option for complete verification
rsync --checksum --ignore-times --partial --progress /data/ disaster-recovery:/backup/

# Optimized combination for large binary files
rsync --checksum --size-only --compress /media-files/ cloud-storage:/assets/

The --checksum --ignore-times combination effectively forces a full checksum comparison on every file, which is useful for forensic backups but extremely resource-intensive.

Problem: Unexpected file transfers despite using these flags
Solution: Check for filesystem-specific behaviors (NTFS timestamps, FAT32 limitations)

# Example diagnostic command
rsync --dry-run --itemize-changes --checksum /source/ dest/ | grep '^[^.]'

Remember that both options interact with rsync's other optimization flags like --size-only and --whole-file in non-obvious ways.

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While both --checksum and --ignore-times modify rsync's default comparison behavior, they operate at different levels of the synchronization process:

# Default rsync behavior (time+size comparison)
rsync -avz source/ destination/

# Using --checksum
rsync -avz --checksum source/ destination/

# Using --ignore-times
rsync -avz --ignore-times source/ destination/

The --checksum option forces rsync to:

• Skip the quick check based on file size and modification time
• Calculate MD5 checksums for all files
• Compare checksums between source and destination

Meanwhile, --ignore-times:

• Ignores modification time differences
• Still performs size comparison
• Uses rsync's delta algorithm for efficient transfers

When to use --checksum:

# Example: Verifying backup integrity
rsync --checksum --delete -av /critical-data/ backup-server:/backups/

When to use --ignore-times:

# Example: Forcing content comparison after timestamp changes
rsync --ignore-times -avz compiled-code/ test-server:/deploy/

Option	CPU Usage	Network Usage	Use Case
--checksum	High (checksum calculation)	Low (only changed files)	Data verification
--ignore-times	Medium	Medium (delta transfer)	Forced comparison

For maximum reliability in critical transfers:

rsync --checksum --ignore-times --partial --progress -avz \
    source-data/ remote-server:/backup/

This combination ensures:

• Complete content verification (--checksum)
• No timestamp assumptions (--ignore-times)
• Resumable transfers (--partial)
• Progress visibility (--progress)