ZPAQ vs ARJ Compression Formats

ZPAQ (2009 - Present)

ZPAQ is an open-source, journaling archiver with extremely high compression ratios, designed by Matt Mahoney. It uses incremental deduplication and supports multiple compression algorithms.

Key Features

Journaling: Maintains complete version history of archived files
Incremental backups: Only stores changes between versions
High compression: Often outperforms other formats in compression ratio
Encryption: Supports AES-256 encryption
Platform support: Windows, Linux, macOS
Open format: Public specification and open-source implementation

ARJ (1991 - Present)

ARJ (Archived by Robert Jung) is a compressed file archive format popular in the DOS era, known for its multi-volume support and relatively fast compression speeds.

Key Features

Multi-volume archives: Splits archives across multiple disks
Fast compression: Faster than ZPAQ in most cases
Solid compression: Compresses files together for better ratios
Password protection: Basic password security
DOS heritage: Originally designed for DOS systems
File recovery: Built-in recovery record feature

Technical Comparison

Feature	ZPAQ	ARJ
Compression Ratio	Excellent (best in class)	Good (typical for its era)
Compression Speed	Very Slow	Fast
Decompression Speed	Slow	Very Fast
Versioning Support	Full journaling support	None
Encryption	AES-256	Basic password protection
Multi-volume Support	No	Yes
Platform Support	Cross-platform	Primarily DOS/Windows
File Size Limit	16 exabytes (theoretical)	2GB (practical limit)

When to Use Each Format

Choose ZPAQ When:

You need maximum compression ratio regardless of time
Versioning and incremental backups are important
Working with very large archives (terabytes+)
Strong encryption is required
Archiving for long-term storage

Choose ARJ When:

You need fast compression/decompression
Working with legacy DOS/Windows systems
Creating multi-volume archives for floppy disks
Compatibility with older systems is required
Basic password protection is sufficient

Conclusion

ZPAQ represents the cutting edge of compression technology with its journaling capabilities and extreme compression ratios, making it ideal for archival purposes where storage space is at a premium and time is less critical.

ARJ remains relevant for its speed and multi-volume capabilities, particularly in legacy environments. While it can't match ZPAQ's compression ratios, it offers much faster operation and better compatibility with older systems.

For modern archival needs where maximum compression is desired, ZPAQ is clearly superior. For quick operations or legacy compatibility, ARJ may still be the better choice.