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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.

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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.

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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
The BZip2 compression pipeline:
  1. Block Division: Input divided into blocks (100-900 KB)
  2. Burrows-Wheeler Transform: Sorts block rotations to group similar characters
  3. Move-to-Front: Converts repeated characters to small numbers
  4. Run-Length Encoding: Compresses runs of zeros
  5. Huffman Coding: Final entropy encoding
This multi-stage approach provides excellent compression while maintaining reasonable speed.

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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

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Compression Parameters

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.
BZip2 uses block size as its primary level parameter:
LevelBlock SizeMemory (Compress)Memory (Decompress)
1100 KB~1.2 MB~400 KB
2200 KB~2.3 MB~800 KB
3300 KB~3.5 MB~1.2 MB
4400 KB~4.6 MB~1.6 MB
5500 KB~5.8 MB~2.0 MB
6600 KB~6.9 MB~2.4 MB
7700 KB~8.1 MB~2.8 MB
8800 KB~9.2 MB~3.2 MB
9900 KB~10.4 MB~3.6 MB
Unlike LZMA, BZip2 compression and decompression both have modest memory requirements.

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Memory Requirements

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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.

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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.

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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)

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Compression Ratio Comparison

MethodCompression RatioSpeed (Compress)
PPMd100% (best)Very Slow (1-2 MB/s)
LZMA2105-110%Slow (2-3 MB/s)
BZip2115-125%Medium (5-7 MB/s)
Deflate140-160%Fast (15-25 MB/s)
BZip2 provides a good middle ground for text compression.
MethodCompression RatioSpeed (Compress)
LZMA2 + BCJ100% (best)Slow (2-3 MB/s)
BZip2120-140%Medium (5-7 MB/s)
Deflate150-180%Fast (15-25 MB/s)
For executables, LZMA2 with BCJ filter is superior to BZip2.
MethodResult
All methodsNo compression or expansion
BZip2, like all compression methods, cannot compress already-compressed data (JPEG, PNG, MP3, etc.). Use Copy method instead.

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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.

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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.

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BZip2 vs Other Methods

FeatureBZip2LZMA2DeflatePPMd
Method ID04 02 022104 01 0803 04 01
Compression RatioGood (7/10)Excellent (9/10)Fair (5/10)Excellent (10/10)
Compression SpeedMedium (5/10)Slow (3/10)Fast (7/10)Very Slow (2/10)
Decompression SpeedMedium (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)
MultithreadingNoYesNoNo
Block-basedYesYesNoNo
StreamingYesYesYesYes
CompatibilityWide7z/XZUniversalLimited

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Use Cases

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/
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
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

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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.

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Parallel BZip2 Variants

7-Zip’s BZip2 implementation is single-threaded, but external tools provide parallel variants:
ToolThreadsSpeed Improvement
bzip21Baseline
pbzip2Multiple3-4x on 4 cores
lbzip2Multiple3-4x 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

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Best Practices

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
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
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.

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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

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See Also

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