Advanced Compression Parameters

The 7-Zip Plug-In supports some advanced compression parameters, which are explained below.

Parameters for the Deflate Compression Algorithm (used in ZIP, GZ, GZIP archive types):

Parameter

Meaning

Fast Bytes

Sets the number of fast bytes. It can be in the range from 3 to 255. Bigger numbers provide slightly better compression strength at the expense of longer compression time. A bigger value of the fast bytes parameter can significantly increase compression ratio when files being compressed contain long identical sequences of bytes.

Number of Passes

Sets the number of passes. It can be in the range from 1 to 4. Bigger numbers provide slightly better compression strength at the expense of longer compression time.

If you use more than one pass, each pass will require additional memory calculated as follows: (128 * Fast Bytes) KB.

Deflate is the most popular compression algorithm, originally introduced by PK-Zip in 1992. Deflate was a very good compression algorithm among its contemporaries, however advances in compression technology have rendered its compression ratios feeble.
 

Parameters for the LZMA Compression Algorithm (used in the 7Z archive type):

Parameter

Meaning

Dictionary Size

Sets the size of the dictionary. The maximum value for the dictionary size is 256MB = 2^28 bytes. A larger dictionary provides for significantly enhanced compression ratios, at the expense of compression speed. For decompressing a file compressed using the LZMA algorithm with dictionary size N you need about N*2 memory available.

Match Finder Method

 

Sets the match finder method. Methods from the Binary Tree group require less memory than methods from the Patricia Tree group. Typically, Binary Tree with 2-3-4 bytes hashing works faster than any Patricia Tree group member, but for some types of files Patricia Tree group members may work faster. Algorithms from the Hash Chain group do not provide good compression ratios. Memory requirements for compression depend on the dictionary size.

The match finder method memory requirements for compression are indicated below. d denotes the size of the dictionary.

Memory Required

Match Finder Method

d*13.5 + 1MB

Binary Tree with 2 bytes hashing.

d*13.5 + 65MB

Binary Tree with 2-3(full) bytes hashing.

d*13.5 + 6MB

Binary Tree with 2-3-4 bytes hashing.

d*13.5 + 34MB

Binary Tree with 2-3-4(big) bytes hashing.

d*26 + 1MB

Patricia Tree with 2-bits nodes, removing.

d*38 + 1MB

Patricia Tree with 2-bits nodes.

d*38 + 77MB

Patricia Tree with 2-bits nodes, 2-3 bytes hashing.

d*62 + 85MB

Patricia Tree with 3-bits nodes, 2-3 bytes hashing.

d*110 + 101MB

Patricia Tree with 4-bits nodes, 2-3 bytes hashing.

d*7.5 + 1MB

Hash Chain with 2-3 bytes hashing.

d*7.5 + 6MB

Hash Chain with 2-3-4 bytes hashing.

Fast Bytes

Sets the number of fast bytes. It can be in the range from 3 to 255. Bigger numbers provide slightly better compression strength at the expense of longer compression time. A bigger value of the fast bytes parameter can significantly increase compression ratio when files being compressed contain long identical sequences of bytes.

The LZMA algorithm is a derivative of the Lempel-Ziv algorithm. It provides very fast decompression (about 10-20 times faster than compression), and the strongest compression available today!
 

Parameters for the PPMd Compression Algorithm (used in the 7Z archive type):

Parameter

Meaning

Memory Size

Sets the size of memory usable by PPMd. More memory results in better compression. PPMd uses the same amount of memory for compression and decompression. The maximum value for the memory size is 2GB = 2^31 bytes.

Model Order

Sets the model order for PPMd. It can be in the range from 2 to 32.

PPMd is a PPM based algorithm. This algorithm is mostly based on Dmitry Shkarin's PPMdH sources. PPMd provides very good compression ratios for plain text files.

 

Finally, you may also use a converter algorithm for 7Z archives. Converter algorithms pre-process files to be compressed, increasing their compressibility. Two converters are available: