d7f258d845
updated documentation and man page
530 lines
23 KiB
Markdown
530 lines
23 KiB
Markdown
zstd(1) -- zstd, zstdmt, unzstd, zstdcat - Compress or decompress .zst files
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============================================================================
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SYNOPSIS
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--------
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`zstd` [*OPTIONS*] [-|_INPUT-FILE_] [-o _OUTPUT-FILE_]
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`zstdmt` is equivalent to `zstd -T0`
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`unzstd` is equivalent to `zstd -d`
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`zstdcat` is equivalent to `zstd -dcf`
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DESCRIPTION
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-----------
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`zstd` is a fast lossless compression algorithm and data compression tool,
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with command line syntax similar to `gzip (1)` and `xz (1)`.
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It is based on the **LZ77** family, with further FSE & huff0 entropy stages.
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`zstd` offers highly configurable compression speed,
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with fast modes at > 200 MB/s per core,
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and strong modes nearing lzma compression ratios.
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It also features a very fast decoder, with speeds > 500 MB/s per core.
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`zstd` command line syntax is generally similar to gzip,
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but features the following differences :
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- Source files are preserved by default.
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It's possible to remove them automatically by using the `--rm` command.
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- When compressing a single file, `zstd` displays progress notifications
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and result summary by default.
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Use `-q` to turn them off.
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- `zstd` does not accept input from console,
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but it properly accepts `stdin` when it's not the console.
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- `zstd` displays a short help page when command line is an error.
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Use `-q` to turn it off.
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`zstd` compresses or decompresses each _file_ according to the selected
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operation mode.
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If no _files_ are given or _file_ is `-`, `zstd` reads from standard input
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and writes the processed data to standard output.
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`zstd` will refuse to write compressed data to standard output
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if it is a terminal : it will display an error message and skip the _file_.
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Similarly, `zstd` will refuse to read compressed data from standard input
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if it is a terminal.
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Unless `--stdout` or `-o` is specified, _files_ are written to a new file
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whose name is derived from the source _file_ name:
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* When compressing, the suffix `.zst` is appended to the source filename to
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get the target filename.
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* When decompressing, the `.zst` suffix is removed from the source filename to
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get the target filename
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### Concatenation with .zst files
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It is possible to concatenate `.zst` files as is.
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`zstd` will decompress such files as if they were a single `.zst` file.
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OPTIONS
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-------
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### Integer suffixes and special values
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In most places where an integer argument is expected,
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an optional suffix is supported to easily indicate large integers.
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There must be no space between the integer and the suffix.
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* `KiB`:
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Multiply the integer by 1,024 (2\^10).
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`Ki`, `K`, and `KB` are accepted as synonyms for `KiB`.
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* `MiB`:
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Multiply the integer by 1,048,576 (2\^20).
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`Mi`, `M`, and `MB` are accepted as synonyms for `MiB`.
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### Operation mode
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If multiple operation mode options are given,
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the last one takes effect.
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* `-z`, `--compress`:
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Compress.
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This is the default operation mode when no operation mode option is specified
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and no other operation mode is implied from the command name
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(for example, `unzstd` implies `--decompress`).
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* `-d`, `--decompress`, `--uncompress`:
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Decompress.
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* `-t`, `--test`:
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Test the integrity of compressed _files_.
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This option is equivalent to `--decompress --stdout` except that the
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decompressed data is discarded instead of being written to standard output.
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No files are created or removed.
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* `-b#`:
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Benchmark file(s) using compression level #
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* `--train FILEs`:
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Use FILEs as a training set to create a dictionary.
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The training set should contain a lot of small files (> 100).
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* `-l`, `--list`:
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Display information related to a zstd compressed file, such as size, ratio, and checksum.
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Some of these fields may not be available.
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This command can be augmented with the `-v` modifier.
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### Operation modifiers
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* `-#`:
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`#` compression level \[1-19] (default: 3)
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* `--fast[=#]`:
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switch to ultra-fast compression levels.
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If `=#` is not present, it defaults to `1`.
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The higher the value, the faster the compression speed,
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at the cost of some compression ratio.
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This setting overwrites compression level if one was set previously.
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Similarly, if a compression level is set after `--fast`, it overrides it.
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* `--ultra`:
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unlocks high compression levels 20+ (maximum 22), using a lot more memory.
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Note that decompression will also require more memory when using these levels.
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* `--long[=#]`:
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enables long distance matching with `#` `windowLog`, if not `#` is not
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present it defaults to `27`.
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This increases the window size (`windowLog`) and memory usage for both the
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compressor and decompressor.
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This setting is designed to improve the compression ratio for files with
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long matches at a large distance.
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Note: If `windowLog` is set to larger than 27, `--long=windowLog` or
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`--memory=windowSize` needs to be passed to the decompressor.
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* `-T#`, `--threads=#`:
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Compress using `#` working threads (default: 1).
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If `#` is 0, attempt to detect and use the number of physical CPU cores.
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In all cases, the nb of threads is capped to ZSTDMT_NBTHREADS_MAX==200.
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This modifier does nothing if `zstd` is compiled without multithread support.
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* `--single-thread`:
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Does not spawn a thread for compression, use a single thread for both I/O and compression.
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In this mode, compression is serialized with I/O, which is slightly slower.
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(This is different from `-T1`, which spawns 1 compression thread in parallel of I/O).
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This mode is the only one available when multithread support is disabled.
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Single-thread mode features lower memory usage.
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Final compressed result is slightly different from `-T1`.
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* `--adapt[=min=#,max=#]` :
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`zstd` will dynamically adapt compression level to perceived I/O conditions.
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Compression level adaptation can be observed live by using command `-v`.
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Adaptation can be constrained between supplied `min` and `max` levels.
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The feature works when combined with multi-threading and `--long` mode.
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It does not work with `--single-thread`.
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It sets window size to 8 MB by default (can be changed manually, see `wlog`).
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Due to the chaotic nature of dynamic adaptation, compressed result is not reproducible.
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_note_ : at the time of this writing, `--adapt` can remain stuck at low speed
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when combined with multiple worker threads (>=2).
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* `--stream-size=#` :
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Sets the pledged source size of input coming from a stream. This value must be exact, as it
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will be included in the produced frame header. Incorrect stream sizes will cause an error.
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This information will be used to better optimize compression parameters, resulting in
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better and potentially faster compression, especially for smaller source sizes.
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* `--size-hint=#`:
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When handling input from a stream, `zstd` must guess how large the source size
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will be when optimizing compression parameters. If the stream size is relatively
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small, this guess may be a poor one, resulting in a higher compression ratio than
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expected. This feature allows for controlling the guess when needed.
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Exact guesses result in better compression ratios. Overestimates result in slightly
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degraded compression ratios, while underestimates may result in significant degradation.
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* `--rsyncable` :
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`zstd` will periodically synchronize the compression state to make the
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compressed file more rsync-friendly. There is a negligible impact to
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compression ratio, and the faster compression levels will see a small
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compression speed hit.
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This feature does not work with `--single-thread`. You probably don't want
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to use it with long range mode, since it will decrease the effectiveness of
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the synchronization points, but your milage may vary.
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* `-D file`:
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use `file` as Dictionary to compress or decompress FILE(s)
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* `--no-dictID`:
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do not store dictionary ID within frame header (dictionary compression).
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The decoder will have to rely on implicit knowledge about which dictionary to use,
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it won't be able to check if it's correct.
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* `-o file`:
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save result into `file` (only possible with a single _INPUT-FILE_)
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* `-f`, `--force`:
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overwrite output without prompting, and (de)compress symbolic links
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* `-c`, `--stdout`:
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force write to standard output, even if it is the console
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* `--[no-]sparse`:
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enable / disable sparse FS support,
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to make files with many zeroes smaller on disk.
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Creating sparse files may save disk space and speed up decompression by
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reducing the amount of disk I/O.
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default: enabled when output is into a file,
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and disabled when output is stdout.
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This setting overrides default and can force sparse mode over stdout.
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* `--rm`:
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remove source file(s) after successful compression or decompression
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* `-k`, `--keep`:
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keep source file(s) after successful compression or decompression.
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This is the default behavior.
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* `-r`:
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operate recursively on directories
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* `--filelist=FILE`
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read a list of files to process as content from `FILE`.
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Format is compatible with `ls` output, with one file per file.
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* `--output-dir-flat[=dir]`:
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resulting files are stored into target `dir` directory,
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instead of same directory as origin file.
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Be aware that this command can introduce name collision issues,
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if multiple files, from different directories, end up having the same name.
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Collision resolution ensures first file with a given name will be present in `dir`,
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while in combination with `-f`, the last file will be present instead.
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* `--format=FORMAT`:
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compress and decompress in other formats. If compiled with
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support, zstd can compress to or decompress from other compression algorithm
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formats. Possibly available options are `zstd`, `gzip`, `xz`, `lzma`, and `lz4`.
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If no such format is provided, `zstd` is the default.
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* `-h`/`-H`, `--help`:
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display help/long help and exit
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* `-V`, `--version`:
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display version number and exit.
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Advanced : `-vV` also displays supported formats.
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`-vvV` also displays POSIX support.
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* `-v`:
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verbose mode
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* `-q`, `--quiet`:
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suppress warnings, interactivity, and notifications.
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specify twice to suppress errors too.
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* `--no-progress`:
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do not display the progress bar, but keep all other messages.
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* `-C`, `--[no-]check`:
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add integrity check computed from uncompressed data (default: enabled)
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* `--`:
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All arguments after `--` are treated as files
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### Restricted usage of Environment Variables
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Using environment variables to set parameters has security implications.
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Therefore, this avenue is intentionally restricted.
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Only `ZSTD_CLEVEL` is supported currently, for setting compression level.
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`ZSTD_CLEVEL` can be used to set the level between 1 and 19 (the "normal" range).
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If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message.
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`ZSTD_CLEVEL` just replaces the default compression level (`3`).
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It can be overridden by corresponding command line arguments.
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DICTIONARY BUILDER
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------------------
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`zstd` offers _dictionary_ compression,
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which greatly improves efficiency on small files and messages.
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It's possible to train `zstd` with a set of samples,
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the result of which is saved into a file called a `dictionary`.
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Then during compression and decompression, reference the same dictionary,
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using command `-D dictionaryFileName`.
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Compression of small files similar to the sample set will be greatly improved.
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* `--train FILEs`:
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Use FILEs as training set to create a dictionary.
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The training set should contain a lot of small files (> 100),
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and weight typically 100x the target dictionary size
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(for example, 10 MB for a 100 KB dictionary).
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Supports multithreading if `zstd` is compiled with threading support.
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Additional parameters can be specified with `--train-fastcover`.
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The legacy dictionary builder can be accessed with `--train-legacy`.
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The cover dictionary builder can be accessed with `--train-cover`.
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Equivalent to `--train-fastcover=d=8,steps=4`.
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* `-o file`:
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Dictionary saved into `file` (default name: dictionary).
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* `--maxdict=#`:
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Limit dictionary to specified size (default: 112640).
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* `-#`:
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Use `#` compression level during training (optional).
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Will generate statistics more tuned for selected compression level,
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resulting in a _small_ compression ratio improvement for this level.
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* `-B#`:
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Split input files in blocks of size # (default: no split)
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* `--dictID=#`:
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A dictionary ID is a locally unique ID that a decoder can use to verify it is
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using the right dictionary.
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By default, zstd will create a 4-bytes random number ID.
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It's possible to give a precise number instead.
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Short numbers have an advantage : an ID < 256 will only need 1 byte in the
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compressed frame header, and an ID < 65536 will only need 2 bytes.
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This compares favorably to 4 bytes default.
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However, it's up to the dictionary manager to not assign twice the same ID to
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2 different dictionaries.
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* `--train-cover[=k#,d=#,steps=#,split=#,shrink[=#]]`:
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Select parameters for the default dictionary builder algorithm named cover.
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If _d_ is not specified, then it tries _d_ = 6 and _d_ = 8.
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If _k_ is not specified, then it tries _steps_ values in the range [50, 2000].
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If _steps_ is not specified, then the default value of 40 is used.
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If _split_ is not specified or split <= 0, then the default value of 100 is used.
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Requires that _d_ <= _k_.
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If _shrink_ flag is not used, then the default value for _shrinkDict_ of 0 is used.
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If _shrink_ is not specified, then the default value for _shrinkDictMaxRegression_ of 1 is used.
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Selects segments of size _k_ with highest score to put in the dictionary.
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The score of a segment is computed by the sum of the frequencies of all the
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subsegments of size _d_.
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Generally _d_ should be in the range [6, 8], occasionally up to 16, but the
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algorithm will run faster with d <= _8_.
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Good values for _k_ vary widely based on the input data, but a safe range is
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[2 * _d_, 2000].
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If _split_ is 100, all input samples are used for both training and testing
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to find optimal _d_ and _k_ to build dictionary.
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Supports multithreading if `zstd` is compiled with threading support.
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Having _shrink_ enabled takes a truncated dictionary of minimum size and doubles
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in size until compression ratio of the truncated dictionary is at most
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_shrinkDictMaxRegression%_ worse than the compression ratio of the largest dictionary.
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Examples:
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`zstd --train-cover FILEs`
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`zstd --train-cover=k=50,d=8 FILEs`
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`zstd --train-cover=d=8,steps=500 FILEs`
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`zstd --train-cover=k=50 FILEs`
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`zstd --train-cover=k=50,split=60 FILEs`
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`zstd --train-cover=shrink FILEs`
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`zstd --train-cover=shrink=2 FILEs`
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* `--train-fastcover[=k#,d=#,f=#,steps=#,split=#,accel=#]`:
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Same as cover but with extra parameters _f_ and _accel_ and different default value of split
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If _split_ is not specified, then it tries _split_ = 75.
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If _f_ is not specified, then it tries _f_ = 20.
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Requires that 0 < _f_ < 32.
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If _accel_ is not specified, then it tries _accel_ = 1.
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Requires that 0 < _accel_ <= 10.
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Requires that _d_ = 6 or _d_ = 8.
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_f_ is log of size of array that keeps track of frequency of subsegments of size _d_.
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The subsegment is hashed to an index in the range [0,2^_f_ - 1].
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It is possible that 2 different subsegments are hashed to the same index, and they are considered as the same subsegment when computing frequency.
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Using a higher _f_ reduces collision but takes longer.
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Examples:
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`zstd --train-fastcover FILEs`
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`zstd --train-fastcover=d=8,f=15,accel=2 FILEs`
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* `--train-legacy[=selectivity=#]`:
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Use legacy dictionary builder algorithm with the given dictionary
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_selectivity_ (default: 9).
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The smaller the _selectivity_ value, the denser the dictionary,
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improving its efficiency but reducing its possible maximum size.
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`--train-legacy=s=#` is also accepted.
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Examples:
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`zstd --train-legacy FILEs`
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`zstd --train-legacy=selectivity=8 FILEs`
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BENCHMARK
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---------
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* `-b#`:
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benchmark file(s) using compression level #
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* `-e#`:
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benchmark file(s) using multiple compression levels, from `-b#` to `-e#` (inclusive)
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* `-i#`:
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minimum evaluation time, in seconds (default: 3s), benchmark mode only
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* `-B#`, `--block-size=#`:
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cut file(s) into independent blocks of size # (default: no block)
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* `--priority=rt`:
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set process priority to real-time
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**Output Format:** CompressionLevel#Filename : IntputSize -> OutputSize (CompressionRatio), CompressionSpeed, DecompressionSpeed
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**Methodology:** For both compression and decompression speed, the entire input is compressed/decompressed in-memory to measure speed. A run lasts at least 1 sec, so when files are small, they are compressed/decompressed several times per run, in order to improve measurement accuracy.
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ADVANCED COMPRESSION OPTIONS
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----------------------------
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### --zstd[=options]:
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`zstd` provides 22 predefined compression levels.
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The selected or default predefined compression level can be changed with
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advanced compression options.
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The _options_ are provided as a comma-separated list.
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You may specify only the options you want to change and the rest will be
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taken from the selected or default compression level.
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The list of available _options_:
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- `strategy`=_strat_, `strat`=_strat_:
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Specify a strategy used by a match finder.
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There are 9 strategies numbered from 1 to 9, from faster to stronger:
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1=ZSTD\_fast, 2=ZSTD\_dfast, 3=ZSTD\_greedy,
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4=ZSTD\_lazy, 5=ZSTD\_lazy2, 6=ZSTD\_btlazy2,
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7=ZSTD\_btopt, 8=ZSTD\_btultra, 9=ZSTD\_btultra2.
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- `windowLog`=_wlog_, `wlog`=_wlog_:
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Specify the maximum number of bits for a match distance.
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The higher number of increases the chance to find a match which usually
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improves compression ratio.
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It also increases memory requirements for the compressor and decompressor.
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The minimum _wlog_ is 10 (1 KiB) and the maximum is 30 (1 GiB) on 32-bit
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platforms and 31 (2 GiB) on 64-bit platforms.
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Note: If `windowLog` is set to larger than 27, `--long=windowLog` or
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`--memory=windowSize` needs to be passed to the decompressor.
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- `hashLog`=_hlog_, `hlog`=_hlog_:
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Specify the maximum number of bits for a hash table.
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Bigger hash tables cause less collisions which usually makes compression
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faster, but requires more memory during compression.
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The minimum _hlog_ is 6 (64 B) and the maximum is 26 (128 MiB).
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- `chainLog`=_clog_, `clog`=_clog_:
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Specify the maximum number of bits for a hash chain or a binary tree.
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Higher numbers of bits increases the chance to find a match which usually
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improves compression ratio.
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It also slows down compression speed and increases memory requirements for
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compression.
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This option is ignored for the ZSTD_fast strategy.
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The minimum _clog_ is 6 (64 B) and the maximum is 28 (256 MiB).
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- `searchLog`=_slog_, `slog`=_slog_:
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Specify the maximum number of searches in a hash chain or a binary tree
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using logarithmic scale.
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More searches increases the chance to find a match which usually increases
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compression ratio but decreases compression speed.
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The minimum _slog_ is 1 and the maximum is 26.
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- `minMatch`=_mml_, `mml`=_mml_:
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Specify the minimum searched length of a match in a hash table.
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Larger search lengths usually decrease compression ratio but improve
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decompression speed.
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The minimum _mml_ is 3 and the maximum is 7.
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- `targetLen`=_tlen_, `tlen`=_tlen_:
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The impact of this field vary depending on selected strategy.
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For ZSTD\_btopt, ZSTD\_btultra and ZSTD\_btultra2, it specifies
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the minimum match length that causes match finder to stop searching.
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A larger `targetLen` usually improves compression ratio
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but decreases compression speed.
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For ZSTD\_fast, it triggers ultra-fast mode when > 0.
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The value represents the amount of data skipped between match sampling.
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Impact is reversed : a larger `targetLen` increases compression speed
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but decreases compression ratio.
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For all other strategies, this field has no impact.
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The minimum _tlen_ is 0 and the maximum is 999.
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- `overlapLog`=_ovlog_, `ovlog`=_ovlog_:
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Determine `overlapSize`, amount of data reloaded from previous job.
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This parameter is only available when multithreading is enabled.
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Reloading more data improves compression ratio, but decreases speed.
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|
|
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The minimum _ovlog_ is 0, and the maximum is 9.
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1 means "no overlap", hence completely independent jobs.
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9 means "full overlap", meaning up to `windowSize` is reloaded from previous job.
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Reducing _ovlog_ by 1 reduces the reloaded amount by a factor 2.
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|
For example, 8 means "windowSize/2", and 6 means "windowSize/8".
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Value 0 is special and means "default" : _ovlog_ is automatically determined by `zstd`.
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In which case, _ovlog_ will range from 6 to 9, depending on selected _strat_.
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- `ldmHashLog`=_lhlog_, `lhlog`=_lhlog_:
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|
Specify the maximum size for a hash table used for long distance matching.
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|
|
|
This option is ignored unless long distance matching is enabled.
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|
|
|
Bigger hash tables usually improve compression ratio at the expense of more
|
|
memory during compression and a decrease in compression speed.
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|
|
|
The minimum _lhlog_ is 6 and the maximum is 26 (default: 20).
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|
|
|
- `ldmMinMatch`=_lmml_, `lmml`=_lmml_:
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|
Specify the minimum searched length of a match for long distance matching.
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|
|
|
This option is ignored unless long distance matching is enabled.
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|
|
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Larger/very small values usually decrease compression ratio.
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|
|
|
The minimum _lmml_ is 4 and the maximum is 4096 (default: 64).
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|
|
|
- `ldmBucketSizeLog`=_lblog_, `lblog`=_lblog_:
|
|
Specify the size of each bucket for the hash table used for long distance
|
|
matching.
|
|
|
|
This option is ignored unless long distance matching is enabled.
|
|
|
|
Larger bucket sizes improve collision resolution but decrease compression
|
|
speed.
|
|
|
|
The minimum _lblog_ is 0 and the maximum is 8 (default: 3).
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|
|
|
- `ldmHashRateLog`=_lhrlog_, `lhrlog`=_lhrlog_:
|
|
Specify the frequency of inserting entries into the long distance matching
|
|
hash table.
|
|
|
|
This option is ignored unless long distance matching is enabled.
|
|
|
|
Larger values will improve compression speed. Deviating far from the
|
|
default value will likely result in a decrease in compression ratio.
|
|
|
|
The default value is `wlog - lhlog`.
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|
|
|
### Example
|
|
The following parameters sets advanced compression options to something
|
|
similar to predefined level 19 for files bigger than 256 KB:
|
|
|
|
`--zstd`=wlog=23,clog=23,hlog=22,slog=6,mml=3,tlen=48,strat=6
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|
|
|
### -B#:
|
|
Select the size of each compression job.
|
|
This parameter is available only when multi-threading is enabled.
|
|
Default value is `4 * windowSize`, which means it varies depending on compression level.
|
|
`-B#` makes it possible to select a custom value.
|
|
Note that job size must respect a minimum value which is enforced transparently.
|
|
This minimum is either 1 MB, or `overlapSize`, whichever is largest.
|
|
|
|
BUGS
|
|
----
|
|
Report bugs at: https://github.com/facebook/zstd/issues
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|
|
AUTHOR
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|
------
|
|
Yann Collet
|