BZip2 Compression

BZip2 is a compression method based on the Burrows-Wheeler Transform (BWT) and Huffman coding. It provides a good balance between compression ratio, speed, and memory usage.

Method ID

ID: 04 02 02 (hex) The BZip2 method is identified by the 3-byte sequence 04 02 02 in 7z archive format.

Note: ZIP archives use a different ID (04 01 0C) for BZip2, but this is not used in 7z. Always use 04 02 02 for 7z archives.

Overview

BZip2 is a widely-used compression algorithm that:

Uses block-sorting (Burrows-Wheeler Transform)
Applies Move-to-Front transform
Compresses with Huffman coding
Provides consistent, predictable compression ratios
Has moderate memory requirements

How BZip2 Works

The BZip2 compression pipeline:

Block Division: Input divided into blocks (100-900 KB)
Burrows-Wheeler Transform: Sorts block rotations to group similar characters
Move-to-Front: Converts repeated characters to small numbers
Run-Length Encoding: Compresses runs of zeros
Huffman Coding: Final entropy encoding

This multi-stage approach provides excellent compression while maintaining reasonable speed.

Algorithm Characteristics

Burrows-Wheeler Transform

Reversible transformation that groups similar bytes together for better compression

Block-Based Processing

Processes data in independent blocks, enabling parallel decompression

Move-to-Front

Exploits locality by moving recently seen characters to front of alphabet

Huffman Coding

Standard entropy coding for final compression stage

Compression Parameters

Block Size

Range: 100 KB to 900 KB
Values: 1-9 (corresponds to 100KB, 200KB, …, 900KB)
Default: 9 (900 KB)Block size determines:

Compression ratio: Larger blocks = better compression
Memory usage: Larger blocks = more memory
Parallelization: Each block can be processed independently

# Compress with 900 KB blocks (default)
7z a -m0=BZip2 archive.7z file.txt

# Compress with 100 KB blocks (lower memory)
7z a -m0=BZip2:100k archive.7z file.txt

# Compress with 500 KB blocks
7z a -m0=BZip2:500k archive.7z file.txt

For most files, the default 900 KB block size provides the best compression. Use smaller blocks only if memory is limited.

Compression Level

BZip2 uses block size as its primary level parameter:

Level	Block Size	Memory (Compress)	Memory (Decompress)
1	100 KB	~1.2 MB	~400 KB
2	200 KB	~2.3 MB	~800 KB
3	300 KB	~3.5 MB	~1.2 MB
4	400 KB	~4.6 MB	~1.6 MB
5	500 KB	~5.8 MB	~2.0 MB
6	600 KB	~6.9 MB	~2.4 MB
7	700 KB	~8.1 MB	~2.8 MB
8	800 KB	~9.2 MB	~3.2 MB
9	900 KB	~10.4 MB	~3.6 MB

Unlike LZMA, BZip2 compression and decompression both have modest memory requirements.

Memory Requirements

Encoding

Memory ≈ (blockSize * 8) + 600 KB

For default 900 KB blocks:

Memory ≈ (900 KB * 8) + 600 KB ≈ 7.8 MB

BZip2 compression typically requires:

Level 1 (100 KB): ~1.2 MB
Level 5 (500 KB): ~5.8 MB
Level 9 (900 KB): ~10.4 MB

Much less than LZMA/LZMA2 which can require hundreds of MB or even GB.

Decoding

Memory ≈ (blockSize * 4) + overhead

For default 900 KB blocks:

Memory ≈ (900 KB * 4) ≈ 3.6 MB

Decompression uses about half the memory of compression, and both are significantly lower than LZMA methods.

Performance Characteristics

Typical performance on modern hardware (Intel Core i7, 3.5 GHz):Compression:

Level 1 (100 KB): ~8-10 MB/s
Level 5 (500 KB): ~6-8 MB/s
Level 9 (900 KB): ~5-7 MB/s

Decompression:

All levels: ~15-25 MB/s

Compression ratio:

Text files: 20-30% of original size
Executable files: 40-60% of original size
Multimedia files: 85-100% of original size (poor)

Compression Ratio Comparison

Text Files

Method	Compression Ratio	Speed (Compress)
PPMd	100% (best)	Very Slow (1-2 MB/s)
LZMA2	105-110%	Slow (2-3 MB/s)
BZip2	115-125%	Medium (5-7 MB/s)
Deflate	140-160%	Fast (15-25 MB/s)

BZip2 provides a good middle ground for text compression.

Binary Executables

Method	Compression Ratio	Speed (Compress)
LZMA2 + BCJ	100% (best)	Slow (2-3 MB/s)
BZip2	120-140%	Medium (5-7 MB/s)
Deflate	150-180%	Fast (15-25 MB/s)

For executables, LZMA2 with BCJ filter is superior to BZip2.

Already Compressed Data

Method	Result
All methods	No compression or expansion

BZip2, like all compression methods, cannot compress already-compressed data (JPEG, PNG, MP3, etc.). Use Copy method instead.

Command Line Usage

# Compress with BZip2 (default 900 KB blocks)
7z a -m0=BZip2 archive.7z file.txt

# Specify block size (100 KB)
7z a -m0=BZip2:100k archive.7z file.txt

# Specify block size (500 KB)
7z a -m0=BZip2:500k archive.7z file.txt

# Maximum compression (900 KB)
7z a -m0=BZip2:900k archive.7z file.txt

# Fast compression (100 KB, less memory)
7z a -m0=BZip2:100k archive.7z folder/

# BZip2 with multiple files
7z a -m0=BZip2 archive.7z *.txt

BZip2 in 7z archives is single-threaded. For parallel compression, use LZMA2 with -mmt option.

BZip2 File Format

Standalone .bz2 files have this structure:

Magic bytes: 'BZ' (0x42 0x5A)
Version: 'h' (0x68) for BZip2
Block size: '1'-'9' (100 KB to 900 KB)
Compressed blocks:
  Block header: 0x314159265359 (48 bits)
  Block data
  ...
End marker: 0x177245385090 (48 bits)
CRC-32: 4 bytes

In 7z archives, BZip2 streams are wrapped in the 7z format and don’t include the standalone file headers.

BZip2 vs Other Methods

Feature	BZip2	LZMA2	Deflate	PPMd
Method ID	04 02 02	21	04 01 08	03 04 01
Compression Ratio	Good (7/10)	Excellent (9/10)	Fair (5/10)	Excellent (10/10)
Compression Speed	Medium (5/10)	Slow (3/10)	Fast (7/10)	Very Slow (2/10)
Decompression Speed	Medium (6/10)	Medium (6/10)	Very Fast (9/10)	Slow (4/10)
Memory (Compress)	Low (~10 MB)	High (~200 MB)	Very Low (~1 MB)	High (~64 MB)
Memory (Decompress)	Low (~4 MB)	Medium (~50 MB)	Very Low (~32 KB)	High (~64 MB)
Multithreading	No	Yes	No	No
Block-based	Yes	Yes	No	No
Streaming	Yes	Yes	Yes	Yes
Compatibility	Wide	7z/XZ	Universal	Limited

Use Cases

When to Use BZip2

BZip2 is ideal for:

Memory-constrained systems: Uses much less RAM than LZMA
Compatibility: Widely supported across platforms
Balanced performance: Good ratio without excessive time
Log files: Excellent for text-based logs
Source code archives: Good compression with reasonable speed
Incremental backups: Predictable compression ratios

# Compress system logs
7z a -m0=BZip2 logs.7z /var/log/*.log

# Archive source code
7z a -m0=BZip2 source.7z project/

When NOT to Use BZip2

Avoid BZip2 for:

Maximum compression needed: Use LZMA2 or PPMd instead
Need for speed: Use Deflate for faster compression/decompression
Large files with multithreading: Use LZMA2 with -mmt
Already compressed data: Will not compress further
Binary executables: LZMA2 with BCJ filter is much better

# Wrong: BZip2 for multimedia
7z a -m0=BZip2 archive.7z videos/*.mp4  # No benefit

# Right: Use Copy or LZMA2
7z a -m0=Copy archive.7z videos/*.mp4
7z a archive.7z videos/*.mp4  # Auto-detect

BZip2 vs LZMA2 Decision Tree

Choose BZip2 if:

Memory < 50 MB available
Need wide compatibility
Prefer faster compression than LZMA

Choose LZMA2 if:

Memory > 100 MB available
Want maximum compression
Can use multithreading
Primarily use 7-Zip/p7zip tools

Choose Deflate if:

Need ZIP compatibility
Speed is critical
Very low memory required

Choose PPMd if:

Compressing text/structured data
Want absolute maximum compression
Speed is not important

Interoperability

BZip2 compressed data in 7z archives uses the standard BZip2 algorithm and is compatible with:

Command-line tools: bzip2, bunzip2, pbzip2
Libraries: libbz2, Apache Commons Compress
Languages: Python (bz2 module), Java, C/C++, Rust
Archive formats: 7z, tar.bz2, .bz2

Data compressed with 7-Zip’s BZip2 can be extracted with standard BZip2 tools and vice versa.

Parallel BZip2 Variants

7-Zip’s BZip2 implementation is single-threaded, but external tools provide parallel variants:

Tool	Threads	Speed Improvement
bzip2	1	Baseline
pbzip2	Multiple	3-4x on 4 cores
lbzip2	Multiple	3-4x on 4 cores

For faster BZip2 compression outside 7-Zip, use pbzip2 or lbzip2 with tar:

# Parallel BZip2 with tar
tar -c folder/ | pbzip2 -9 -p4 > archive.tar.bz2

# Decompress in parallel
pbzip2 -d -p4 archive.tar.bz2

Best Practices

Block Size Selection

General recommendations:

Small files (< 100 KB): Use 100-300 KB blocks
Medium files (100 KB - 10 MB): Use 500-700 KB blocks
Large files (> 10 MB): Use 900 KB blocks (default)
Very limited memory: Use 100-200 KB blocks

# Auto-select based on file size
if [ $(stat -f%z "$file") -lt 100000 ]; then
  7z a -m0=BZip2:100k archive.7z "$file"
else
  7z a -m0=BZip2 archive.7z "$file"  # Default 900k
fi

Testing Compression Ratio

Compare BZip2 with other methods:

# Test different methods
7z a -m0=BZip2 bzip2.7z file.txt
7z a -m0=LZMA2 lzma2.7z file.txt
7z a -m0=Deflate deflate.7z file.txt

# Compare sizes
ls -lh *.7z

# Test decompression speed
time 7z x bzip2.7z
time 7z x lzma2.7z
time 7z x deflate.7z

Solid vs Non-Solid Archives

BZip2 can be used in both solid and non-solid archives:

# Solid archive (better compression, slower random access)
7z a -m0=BZip2 -ms=on archive.7z folder/

# Non-solid archive (worse compression, faster random access)
7z a -m0=BZip2 -ms=off archive.7z folder/

For BZip2, the difference between solid and non-solid is less dramatic than with LZMA, since BZip2 already uses block-based compression.

Error Handling

BZip2 is robust and provides good error detection:

CRC-32 checksums: Each block has a CRC for integrity
Magic numbers: Easy detection of format errors
Independent blocks: Corruption limited to single block

If a BZip2 compressed file is corrupted:

Only the corrupted block(s) will be affected
Other blocks may still be recoverable
Use bzip2recover tool to attempt recovery of .bz2 files

Getting Started

Command Reference

Compression Methods

Archive Formats

Advanced Usage

BZip2 Compression

Method ID

Overview

Algorithm Characteristics

Burrows-Wheeler Transform

Block-Based Processing

Move-to-Front

Huffman Coding

Compression Parameters

Memory Requirements

Encoding

Decoding

Performance Characteristics

Compression Ratio Comparison

Command Line Usage

BZip2 File Format

BZip2 vs Other Methods

Use Cases

Interoperability

Parallel BZip2 Variants

Best Practices

Error Handling

See Also

​Method ID

​Overview

​Algorithm Characteristics

Burrows-Wheeler Transform

Block-Based Processing

Move-to-Front

Huffman Coding

​Compression Parameters

​Memory Requirements

​Encoding

​Decoding

​Performance Characteristics

​Compression Ratio Comparison

​Command Line Usage

​BZip2 File Format

​BZip2 vs Other Methods

​Use Cases

​Interoperability

​Parallel BZip2 Variants

​Best Practices

​Error Handling

​See Also

Method ID

Overview

Algorithm Characteristics

Compression Parameters

Memory Requirements

Encoding

Decoding

Performance Characteristics

Compression Ratio Comparison

Command Line Usage

BZip2 File Format

BZip2 vs Other Methods

Use Cases

Interoperability

Parallel BZip2 Variants

Best Practices

Error Handling

See Also