-m (Set compression Method) switch

Specifies the compression method.

Syntax

-m<method_parameters>

The format for this switch depends on the archive type.

The updating of solid .7z archives can be slow, since it can require some recompression.

Example: 

        s=100f10m

set solid mode with 100 files & 10 MB limits per one solid block.

qs=[off | on]

Enables or disables sorting files by type in solid archives. The default mode is qs=off.

Old versions of 7-Zip (before version 15.06) used file sorting "by type" ("by extension").

New versions of 7-Zip (starting from version 15.06) support two sorting orders:

You can get big difference in compression ratio for different sorting methods, if dictionary size is smaller than total size of files. If there are similar files in different folders, the sorting "by type" can provide better compression ratio in some cases.

Note that sorting "by type" has some drawbacks. For example, NTFS volumes use sorting order "by name", so if an archive uses another sorting, then the speed of some operations for files with unusual order can fall on HDD devices (HDDs have low speed for "seek" operations).

If "qs" mode provides much better compression ratio than default "qs-" mode, you still can increase compression ratio for "qs-" mode by increasing of dictionary size.

If you think that unusual file order is not problem for you, and if better compression ratio with small dictionary is more important for you, use "qs" mode.

Note: There are some files (for example, executable files), that are compressed with additional filter. 7-Zip can't use different compression methods in one solid block, so 7-zip can create several groups of files that don't follow "by name" order in "qs-" mode, but files inside each group are still sorted by name in "qs-" mode.

f=[off | on | FilterID]

Enables or disables compression filters. The default mode is f=on, when 7-zip uses filter only for executable files: dll, exe, ocx, sfx, sys. It uses BCJ2 filter in Ultra mode and BCJ filter in other modes. If f=FilterID if specified, 7-zip uses specified filter for all files. FilterID can be: Delta:{N}, BCJ, BCJ2, ARM, ARMT, IA64, PPC, SPARC.

hc=[off | on]

Enables or disables archive header compressing. The default mode is hc=on. If archive header compressing is enabled, the archive header will be compressed with LZMA method.

he=[off | on]

Enables or disables archive header encryption. The default mode is he=off.

b{C1}[s{S1}]:{C2}[s{S2}]

Binds output stream S1 in coder C1 with input stream S2 in coder C2. If stream number is not specified, stream with number 0 will be used.

Usally coder has one input stream and one output stream. In 7z some coders can have multiple input and output streams.

For example, encoder has one input stream and four output streams.

mt=[off | on | {N}]

Sets multithread mode. If you have a multiprocessor or multicore system, you can get a increase with this switch. 7-Zip supports multithread mode only for LZMA / LZMA2 compression and BZip2 compression / decompression. If you specify {N}, for example mt=4, 7-Zip tries to use 4 threads. LZMA compression uses only 2 threads.

{N}={MethodID}[:param1][:param2] ... [:paramN]

Sets compression method. You can use any number of methods. The default method is LZMA2.

{N} sets the index number of method in methods chain. Numbers must begin from 0. Methods that have smaller numbers will be used before others.

Parameters must be in one of the following forms:

Supported methods:

MethodID Description
LZ-based algorithm
LZMA-based algorithm
Dmitry Shkarin's PPMdH with small changes
BZip2 BWT algorithm
Deflate LZ+Huffman
Copy No compression

Supported filters:

MethodID Description
Delta filter
BCJ converter for x86 executables
converter for x86 executables (version 2)
ARM converter for ARM (little endian) executables
ARMT converter for ARM Thumb (little endian) executables
IA64 converter for IA-64 executables
PPC converter for PowerPC (big endian) executables
SPARC converter for SPARC executables

Filters increase the compression ratio for some types of files. Filters must be used with one of the compression method (for example, BCJ + LZMA).

LZMA

LZMA is an algorithm based on Lempel-Ziv algorithm. It provides very fast decompression (about 10-20 times faster than compression). Memory requirements for compression and decompression also are different (see switch for details).

Parameter Default Description
1 Sets compressing mode
24 Sets Dictionary size
bt4 Sets Match Finder
32 Sets number of Fast Bytes
32 Sets Number of Cycles for Match Finder
3 Sets number of Literal Context bits - [0, 8]
0 Sets number of Literal Pos bits - [0, 4]
2 Set number of Pos Bits - [0, 4]
a=[0|1]

Sets compression mode: 0 = fast, 1 = normal. Default value is 1.

d={Size}[b|k|m|g]

Sets Dictionary size for LZMA. You must specify the size in bytes, kilobytes, or megabytes. The maximum value for dictionary size is 1536 MB, but 32-bit version of 7-Zip allows to specify up to 128 MB dictionary. Default values for LZMA are 24 (16 MB) in normal mode, 25 (32 MB) in maximum mode (-mx=7) and 26 (64 MB) in ultra mode (-mx=9). If you do not specify any symbol from the set [b|k|m|g], the dictionary size will be calculated as DictionarySize = 2^Size bytes. For decompressing a file compressed by LZMA method with dictionary size N, you need about N bytes of memory (RAM) available.

mf={MF_ID}

Sets Match Finder for LZMA. Default method is bt4. Algorithms from hc* group don't provide a good compression ratio, but they often work pretty fast in combination with fast mode (a=0). Memory requirements depend on dictionary size (parameter "d" in table below).

MF_ID Dictionary Memory Usage Description
bt2 9.5 * d + 4 MB Binary Tree 2 bytes hashing
bt3 11.5 * d 3 bytes hashing
bt4 64 KB ... 48 MB 11.5 * d 4 bytes hashing
64 MB ... 1024 MB 10.5 * d
hc4 64 KB ... 48 MB 7.5 * d Hash Chain
64 MB ... 1024 MB 6.5 * d

Note: Your operation system also needs some amount of physical memory for internal purposes. So keep at least 32MB of physical memory unused.

fb={N}

Sets number of fast bytes for LZMA. It can be in the range from 5 to 273. The default value is 32 for normal mode and 64 for maximum and ultra modes. Usually, a big number gives a little bit better compression ratio and slower compression process.

mc={N}

Sets number of cycles (passes) for match finder. It can be in range from 0 to 1000000000. Default value is (16 + number_of_fast_bytes / 2) for BT* match finders and (8 + number_of_fast_bytes / 4) for HC4 match finder. If you specify mc=0, LZMA will use default value. Usually, a big number gives a little bit better compression ratio and slower compression process. For example, mf=HC4 and mc=10000 can provide almost the same compression ratio as mf=BT4.

lc={N}

Sets the number of literal context bits (high bits of previous literal). It can be in range from 0 to 8. Default value is 3. Sometimes lc=4 gives gain for big files.

lp={N}

Sets the number of literal pos bits (low bits of current position for literals). It can be in the range from 0 to 4. The default value is 0. The lp switch is intended for periodical data when the period is equal to 2^value (where lp=value). For example, for 32-bit (4 bytes) periodical data you can use lp=2. Often it's better to set lc=0, if you change lp switch.

pb={N}

Sets the number of pos bits (low bits of current position). It can be in the range from 0 to 4. The default value is 2. The pb switch is intended for periodical data when the period is equal 2^value (where lp=value).

LZMA2

LZMA2 is modified version of LZMA. it provides the following advantages over LZMA:

Parameter Default Description
dictSize * 4 Sets Chunk size

If you don't specify ChunkSize, LZMA2 sets it to max(DictionarySize, min(256M, max(1M, DictionarySize * 4))).

LZMA2 also supports all LZMA parameters, but lp+lc cannot be larger than 4.

LZMA2 uses: 1 thread for each chunk in x1 and x3 modes; and 2 threads for each chunk in x5, x7 and x9 modes. If LZMA2 is set to use only such number of threads required for one chunk, it doesn't split stream to chunks. So you can get different compression ratio for different number of threads. You can get the best compression ratio, when you use 1 or 2 threads.

PPMd

PPMd is a PPM-based algorithm. This algorithm is mostly based on Dmitry Shkarin's PPMdH source code. PPMd provides very good compression ratio for plain text files. There is no difference between compression speed and decompression speed. Memory requirements for compression and decompression also are the same.

Parameter Default Description
24 Sets size of used memory for PPMd.
6 Sets model order for PPMd.
mem={Size}[b|k|m|g]

Sets the size of memory used for PPMd. You must specify the size in bytes, kilobytes, or megabytes. The maximum value is 2GB = 2^31 bytes. The default value is 24 (16MB). If you do not specify any symbol from the set [b|k|m|g], the memory size will be calculated as (2^Size) bytes. PPMd uses the same amount of memory for compression and decompression.

o={Size}

Sets the model order for PPMd. The size must be in the range [2,32]. The default value is 6.

BCJ2

BCJ2 is a Branch converter for 32-bit x86 executables (version 2). It converts some branch instructions for increasing further compression.

A BCJ2 encoder has one input stream and four output streams:

If LZMA is used, the size of the dictionary for streams s1 and s2 can be much smaller (512 KB is enough for most cases) than the dictionary size for stream s0.

Parameters:

d={Size}[b|k|m|g]

Sets section size for BCJ2 filter. Default section size is 64 MB. If you do not specify any symbol from the set [b|k|m|g], the section size will be calculated as SectionSize = 2^Size bytes. This parameter doesn't affect memory consumption. Compression ratio is better, if the section size is equal or slightly larger than size of largest execution section in file. Example: f=BCJ2:d9M, if largest executable section in files is smaller than 9 MB.

Delta

It's possible to set delta offset in bytes. For example, to compress 16-bit stereo WAV files, you can set "0=Delta:4". Default delta offset is 1.

XZ

XZ supports only LZMA2 codec now. The switches are similar to switches for 7z format.

Parameter Default Description
5 Sets level of compression
Sets compression filter. FilterID: Delta:{N}, BCJ, ARM, ARMT, IA64, PPC, SPARC
LZMA2 Sets compression method: LZMA2:[param1]:[param2]:[...]
on Sets multithreading mode
off Sets solid mode.
s=[off | on | [{N}b | {N}k | {N}m | {N}g)]

Enables or disables solid mode. The default mode is s=off. In solid mode, there is only one block per file or stream.

{N}b | {N}k | {N}m | {N}g Set a limit for the total size of a solid block in bytes

If size of solid block is not specified, default value of solid block size will be calculated, that depends from "compression level" and "dictionary size":

dictionary_size Default solid block size
smaller than 256 KB 1 MB
256 KB - 64 MB dictionary_size * 4
64 MB - 256 MB 256 MB
larger than 256 MB dictionary_size

block size must be equal or large than dictionary size.

If you use multiple blocks:

Note: xz uses: 1 thread for each block in x1 and x3 modes; and 2 threads for each block in x5, x7 and x9 modes. If xz is set to use only such number of threads required for one block, it doesn't split stream to blocks. So you can get different compression ratio for different number of threads. You can get the best compression ratio, when you use 1 thread (for x1 and x3 modes) or 2 threads (for x5, x7 and x9 modes).

Note: each xz block contains LZMA2 stream of data. And LZMA2 also can be divided to independent blocks (chunks). The difference between xz blocks and LZMA2 blocks, that each xz block contains also checksum (crc or sha), and there is index record at the end of xz stream that points to each xz block. 7-Zip by default uses xz blocks. But it's possible to specify the mode when it will use one xz block, and multiple LZMA2 blocks instead.

Examples: 

        s=16m

use 16 MB blocks. 

        s

use one solid xz block per file. 

        s 0c16m

use one solid xz block per file and 16 MiB LZMA2 blocks.

WIM

Parameter Default Description
Sets image number.
is=[off | on] off Show image number in paths.

If image number is specified, 7-Zip works only with that image inside WIM archive. Other images will be not changed. By default 7-Zip doesn't show image number, if there is only one image in WIM archive, or if image number is specified. But if the switch "is" specified, 7-Zip shows image number.

Examples

7z a archive.zip *.jpg -mx0

adds *.jpg files to archive.zip archive without compression. 

7z a archive.7z *.exe *.dll -m0=BCJ -m1=LZMA:d=21

adds *.exe and *.dll files to solid archive archive.7z using LZMA method with 2 MB dictionary and BCJ filter. 

7z a archive.7z a.tar -mf=BCJ2 -mx

adds a.tar files to archive archive.7z using BCJ2 filter. 

7z a archive.7z *.wav -mf=Delta:4

adds *.wav files to archive archive.7z using Delta:4 filter. 

7z a a.7z *.exe *.dll -m0=BCJ2 -m1=LZMA:d25 -m2=LZMA:d19 -m3=LZMA:d19 -mb0:1 -mb0s1:2 -mb0s2:3

adds *.exe and *.dll files to archive a.7z using BCJ2 filter, LZMA with 32 MB dictionary for main output stream (s0), and LZMA with 512 KB dictionary for s1 and s2 output streams of BCJ2. 

7z a archive.7z *.txt -m0=PPMd

adds *.txt files to archive archive.7z using PPMd method. 

7z a a.tar.xz a.tar -mf=bcj -mx

adds a.tar files to archive a.tar.xz using BCJ filter.

Commands that can be used with this switch

, , , ,

See also

Switches: ,