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Solid archiving is a compression method that treats all files as a single continuous data stream, achieving significantly better compression ratios.

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What are Solid Archives?

In a solid archive, files are compressed together as one continuous block:
  • All files are treated as a single data stream
  • Compression dictionary is shared across files
  • Similar data patterns across files are efficiently compressed
  • Maximum compression ratio is achieved

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Solid vs Non-Solid

FeatureSolid ArchiveNon-Solid Archive
Compression ratioExcellent (10-50% better)Good
Random accessSlowerFast
Update speedSlowerFast
Memory usageHigherLower
Best forBackups, distributionFrequent updates

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Creating Solid Archives

# Enable solid mode
7z a -ms=on archive.7z files/

# With maximum compression
7z a -ms=on -mx=9 archive.7z files/
Solid mode is enabled by default for 7z archives. Use -ms=off to disable it.

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Solid Block Size

You can control the solid block size for better balance between compression and access speed: 
# 1MB solid blocks
7z a -ms=1m archive.7z files/

# 16MB solid blocks
7z a -ms=16m archive.7z files/

# 1GB solid blocks (maximum compression)
7z a -ms=1g archive.7z files/

# Unlimited (all files in one block)
7z a -ms=on archive.7z files/

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Choosing Block Size

Best for: Archives with frequent random access
7z a -ms=16m archive.7z files/
  • Faster extraction of individual files
  • Lower memory usage
  • Slightly worse compression

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Compression Example

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Non-Solid Archive

# Compress files individually
7z a -ms=off archive.7z files/
Each file is compressed independently:
  • file1.txt → LZMA → compressed block 1
  • file2.txt → LZMA → compressed block 2
  • file3.txt → LZMA → compressed block 3

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Solid Archive

# Compress all files together
7z a -ms=on archive.7z files/
All files compressed as one stream:
  • file1.txt + file2.txt + file3.txt → LZMA → compressed block
Compression improvement example:
  • 1000 text files (1 KB each) = 1 MB total
  • Non-solid: ~400 KB (40% compression)
  • Solid: ~100 KB (90% compression)
The improvement comes from sharing the compression dictionary across similar files.

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Memory Requirements

Solid compression requires more memory:

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Compression Memory

Memory = DictionarySize × (10 to 20)
Example with 64 MB dictionary:
  • Non-solid: ~100 MB
  • Solid: ~1.2 GB

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Decompression Memory

Memory = DictionarySize + SolidBlockSize
Example:
  • Dictionary: 64 MB
  • Solid block: 512 MB
  • Required memory: ~576 MB

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Extraction Performance

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Extracting Single File

Non-solid: 
# Fast - only decompress one file
7z x archive.7z specific-file.txt
Solid: 
# Slower - must decompress all files before the target
7z x archive.7z specific-file.txt

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Extracting All Files

Both solid and non-solid have similar performance when extracting all files: 
# Extract everything
7z x archive.7z

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Solid Archive Organization

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By File Type

Organize files by type for better compression: 
# Group similar files
7z a -ms=on archive.7z *.txt *.log *.xml *.json

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By Extension

Use solid groups to compress similar files together: 
# Solid blocks per extension
7z a -ms=e archive.7z files/
This creates separate solid blocks for each file extension.

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Custom Sorting

# Sort by extension then name
7z a -ms=on archive.7z -stl files/

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Solid Archives with Filters

Combine solid compression with BCJ filters for executables: 
# Solid archive with x86 filter
7z a -ms=on -mf=BCJ archive.7z *.exe *.dll
This provides excellent compression for collections of similar executables.

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Best Practices

Use solid archives for:
  • Backup archives
  • Software distribution
  • Large collections of similar files
  • Long-term storage
  • Maximum compression priority
Avoid solid archives for:
  • Frequently updated archives
  • Archives needing random access
  • Low-memory systems
  • Incremental backups

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Advanced Examples

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Maximum Compression Backup

7z a -t7z -mx=9 -md=64m -mfb=273 -ms=on -mmt=on backup.7z /data/

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Partly-Solid Archive

# 512 KB solid blocks
7z a -ms=512k -mx=9 archive.7z files/
Balances compression ratio and extraction speed.

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Solid Archive with Header Encryption

7z a -ms=on -mhe=on -p archive.7z files/
Maximum compression with full encryption.

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Implementation Details

Solid compression in 7-Zip:
  • Implemented in CPP/7zip/Archive/7z/7zHandler.cpp
  • Solid block structure in CPP/7zip/Archive/7z/7zFolderOutStream.cpp
  • Memory management in CPP/7zip/Common/
From DOC/readme.txt (lines 36-42): 
If you have big number of files in archive, and you need fast extracting,
you can use partly-solid archives:

  7za.exe a archive.7z *.htm -ms=512K -r -mx -m0fb=255 -m0d=512K

In that example 7-Zip will use 512KB solid blocks. So it needs to decompress
only 512KB for extracting one file from such archive.

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See Also

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