When transferring 50GB of small files between machines, the bottleneck typically lies in network throughput rather than disk I/O. The critical factors to consider are:

• Compression ratio efficiency
• CPU overhead during transfer
• Potential for incremental transfers
• Handling of existing files

rsync's -z flag uses zlib compression on the fly during transfer. The key implementation details:

/* Simplified rsync compression logic */
void send_compressed_data(int fd, char *buf, size_t len) {
    z_stream zs;
    deflateInit(&zs, Z_DEFAULT_COMPRESSION);
    zs.next_in = (Bytef *)buf;
    zs.avail_in = len;
    
    do {
        char out[CHUNK];
        zs.next_out = (Bytef *)out;
        zs.avail_out = CHUNK;
        deflate(&zs, Z_SYNC_FLUSH);
        write(fd, out, CHUNK - zs.avail_out);
    } while (zs.avail_out == 0);
    
    deflateEnd(&zs);
}

Testing with a 50GB dataset of web server logs (mostly text):

For most small-file scenarios:

# Best general-purpose approach
rsync -azP --partial /source/ user@dest:/target/

# When dealing with highly compressible data
rsync -azP --compress-level=6 /source/ user@dest:/target/

# Alternative for subsequent syncs
rsync -aP --inplace /source/ user@dest:/target/

For maximum throughput on high-speed networks:

1. Use parallel rsync for small files:

   find /source/ -type f | xargs -n1 -P8 -I% rsync -az % user@dest:/target/

2. Consider zstd compression if available:

   rsync -az --compress-choice=zstd --compress-level=3 /source/ user@dest:/target/

3. Network tuning:

   rsync -azP --sockopts="SOL_SOCKET,SO_SNDBUF=4194304" /source/ user@dest:/target/

Pre-compression makes sense when:

• Source and destination have fast disks but slow network
• Data needs archiving anyway
• Dealing with already compressed formats (JPEG, ZIP, etc.)

Example batch compression script:

#!/bin/bash
# Compress all text files while preserving structure
find /data/ -type f -name "*.log" -exec gzip -k {} \;
rsync -aP /data/ user@remote:/backup/
ssh user@remote "find /backup/ -name "*.gz" -exec gunzip {} \;"

When dealing with massive data transfers (like your 50GB collection of small files), bandwidth and transfer time become critical factors. The fundamental question revolves around compression efficiency versus transfer optimization. Let's break down the technical considerations.

The -z flag in rsync enables compression during transfer, using zlib (similar to gzip) at default level 6. Key technical aspects:

• Compression happens on-the-fly during transfer
• Only compresses the file data (not metadata or delta information)
• Adds CPU overhead on both ends

# Basic rsync with compression
rsync -azP /source/directory/ user@remote:/destination/

Manually compressing before transfer involves:

# Create archive
tar -czvf archive.tar.gz /source/directory/

# Transfer (no compression needed)
rsync -aP archive.tar.gz user@remote:/destination/

# On remote:
tar -xzvf archive.tar.gz

Testing with 50GB of mixed small files (1KB-10MB):

For maximum efficiency with small files:

# Create compressed archive with parallel processing
tar -cf - /source/directory/ | pigz -9 -p 8 > archive.tar.gz

# Transfer with checksum verification
rsync -aP --checksum archive.tar.gz remote:/destination/

# Parallel extraction
ssh remote "pigz -dc archive.tar.gz | tar -xf -"

On high-latency connections (>50ms), the pre-compression method wins due to:

• Fewer round trips
• Better compression ratios (can use higher levels)
• Elimination of rsync's delta computation overhead

rsync -z becomes preferable when:

• You need to frequently sync changing files
• Source/destination have asymmetric CPU power
• Working with already compressed formats (JPEG, ZIP, etc)

For your 50GB one-time transfer of small files:

1. Use parallel compression (pigz/pbzip2 with 4-8 threads)
2. Transfer the single archive with basic rsync
3. Parallel decompress on target

This typically provides 15-25% faster transfer than rsync -z alone.