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What is solid compression
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FAQ / ARCHIVE MANAGER > SOLID MODE COMPRESSION
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What is solid compression



SUGGESTIONS FOR BEST USE OF SOLID MODE DATA COMPRESSION

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Solid archive definition


Solid compression is an option meant to improve compression ratio providing a wider context for compression algorithm while compressing multiple files, which are treated as a single block rather than as separate entities.
The ideas behind solid compression are simple and effective:
  • when multiple files are processed as a single block (especially similar files, i.e. same type / extension, or even revisions of the same file), it is easier to find redundant data between the files of the group, an advantage for the compression algorithm improving efficiency of compressed representation of the data, better than treating each file separately
  • when many small files are processed as a single block, overhead content (marker of file begin/end, checksum, table of content) is written only once rather than once per file, saving extra bytes of size for each input object - a double advantage when compressing a large number of small files, which would normally provide few context to optimize compression and will add multiple overhead content to output archive.
Solid mode is available as option for some archival formats, like in 7Z and RAR standards.
A form of solid compression is used in compressed TAR files (TAR.GZ, TAR.BZ2, TGZ, TXZ...): the data is initially added to a single container, the uncompressed tar archive, then the tar file is compressed as a single block i.e. with GZip, BZip2, or LMZA algorithm.

Main drawbacks of solid compression are:

  • Speed performances penalities on some operations: the context information is needed both during compression / extraction to preserve the advantages of solid compression. So, the partial selective extraction (a single file or group of file rather than the whole archive) from a solid archive, or adding or deleting files to existing archive, and updating existing files in archive needs more time on a solid archive because all the relevant context data (usually defined "solid block") must be parsed, making the process significantly slower than adding / extracting data from a non solid archive
  • for the very same reason, a damage to data in any part of the archive may make all the content after that point non-usable for lack of context information needed for extraction operation, while data corruption in non solid archive usually harms only the data of a single file, with the disadvantage of solid archive being usually more severely damaged by even small loss / corruption of data.

Role of solid block size to improve compression ratio


To mitigate those disadvantages, 7Z format allows to choose the block size to be used for solid mode operation (the "window" of data context that is parsed by the compression / extraction algorithm) minimizing drawbacks bot in terms of overhead during extraction, and possible impact of data corruption.
For the very same reason reducing solid block size reduces potential benefices and compression ratio improvements, providing a smaller context window for the compressor algorithm.

Solid block "size" can be defined by size in bytes, or by number of files in a block; another strategy are blocks separated by file extension, trying to provide more homogeneous data to each single block of the archive.

Read more: solid compressionconsolidate data definition on Wikipedia.


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