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zstd/programs/zstd.1.md
Yann Collet a57d43d4d4 updated documentation of targetLength
2018-03-12 11:35:01 -07:00

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zstd(1) -- zstd, zstdmt, unzstd, zstdcat - Compress or decompress .zst files
============================================================================
SYNOPSIS
--------
`zstd` [*OPTIONS*] [-|_INPUT-FILE_] [-o _OUTPUT-FILE_]
`zstdmt` is equivalent to `zstd -T0`
`unzstd` is equivalent to `zstd -d`
`zstdcat` is equivalent to `zstd -dcf`
DESCRIPTION
-----------
`zstd` is a fast lossless compression algorithm and data compression tool,
with command line syntax similar to `gzip (1)` and `xz (1)`.
It is based on the **LZ77** family, with further FSE & huff0 entropy stages.
`zstd` offers highly configurable compression speed,
with fast modes at > 200 MB/s per code,
and strong modes nearing lzma compression ratios.
It also features a very fast decoder, with speeds > 500 MB/s per core.
`zstd` command line syntax is generally similar to gzip,
but features the following differences :
- Source files are preserved by default.
It's possible to remove them automatically by using the `--rm` command.
- When compressing a single file, `zstd` displays progress notifications
and result summary by default.
Use `-q` to turn them off.
- `zstd` does not accept input from console,
but it properly accepts `stdin` when it's not the console.
- `zstd` displays a short help page when command line is an error.
Use `-q` to turn it off.
`zstd` compresses or decompresses each _file_ according to the selected
operation mode.
If no _files_ are given or _file_ is `-`, `zstd` reads from standard input
and writes the processed data to standard output.
`zstd` will refuse to write compressed data to standard output
if it is a terminal : it will display an error message and skip the _file_.
Similarly, `zstd` will refuse to read compressed data from standard input
if it is a terminal.
Unless `--stdout` or `-o` is specified, _files_ are written to a new file
whose name is derived from the source _file_ name:
* When compressing, the suffix `.zst` is appended to the source filename to
get the target filename.
* When decompressing, the `.zst` suffix is removed from the source filename to
get the target filename
### Concatenation with .zst files
It is possible to concatenate `.zst` files as is.
`zstd` will decompress such files as if they were a single `.zst` file.
OPTIONS
-------
### Integer suffixes and special values
In most places where an integer argument is expected,
an optional suffix is supported to easily indicate large integers.
There must be no space between the integer and the suffix.
* `KiB`:
Multiply the integer by 1,024 (2\^10).
`Ki`, `K`, and `KB` are accepted as synonyms for `KiB`.
* `MiB`:
Multiply the integer by 1,048,576 (2\^20).
`Mi`, `M`, and `MB` are accepted as synonyms for `MiB`.
### Operation mode
If multiple operation mode options are given,
the last one takes effect.
* `-z`, `--compress`:
Compress.
This is the default operation mode when no operation mode option is specified
and no other operation mode is implied from the command name
(for example, `unzstd` implies `--decompress`).
* `-d`, `--decompress`, `--uncompress`:
Decompress.
* `-t`, `--test`:
Test the integrity of compressed _files_.
This option is equivalent to `--decompress --stdout` except that the
decompressed data is discarded instead of being written to standard output.
No files are created or removed.
* `-b#`:
Benchmark file(s) using compression level #
* `--train FILEs`:
Use FILEs as a training set to create a dictionary.
The training set should contain a lot of small files (> 100).
* `-l`, `--list`:
Display information related to a zstd compressed file, such as size, ratio, and checksum.
Some of these fields may not be available.
This command can be augmented with the `-v` modifier.
### Operation modifiers
* `-#`:
`#` compression level \[1-19] (default: 3)
* `--ultra`:
unlocks high compression levels 20+ (maximum 22), using a lot more memory.
Note that decompression will also require more memory when using these levels.
* `--long[=#]`:
enables long distance matching with `#` `windowLog`, if not `#` is not
present it defaults to `27`.
This increases the window size (`windowLog`) and memory usage for both the
compressor and decompressor.
This setting is designed to improve the compression ratio for files with
long matches at a large distance.
Note: If `windowLog` is set to larger than 27, `--long=windowLog` or
`--memory=windowSize` needs to be passed to the decompressor.
* `--fast[=#]`:
switch to ultra-fast compression levels.
If `=#` is not present, it defaults to `1`.
The higher the value, the faster the compression speed,
at the cost of some compression ratio.
This setting overwrites compression level if one was set previously.
Similarly, if a compression level is set after `--fast`, it overrides it.
* `-T#`, `--threads=#`:
Compress using `#` working threads (default: 1).
If `#` is 0, attempt to detect and use the number of physical CPU cores.
In all cases, the nb of threads is capped to ZSTDMT_NBTHREADS_MAX==200.
This modifier does nothing if `zstd` is compiled without multithread support.
* `--single-thread`:
Does not spawn a thread for compression, use caller thread instead.
This is the only available mode when multithread support is disabled.
In this mode, compression is serialized with I/O.
(This is different from `-T1`, which spawns 1 compression thread in parallel of I/O).
Single-thread mode also features lower memory usage.
* `-D file`:
use `file` as Dictionary to compress or decompress FILE(s)
* `--nodictID`:
do not store dictionary ID within frame header (dictionary compression).
The decoder will have to rely on implicit knowledge about which dictionary to use,
it won't be able to check if it's correct.
* `-o file`:
save result into `file` (only possible with a single _INPUT-FILE_)
* `-f`, `--force`:
overwrite output without prompting, and (de)compress symbolic links
* `-c`, `--stdout`:
force write to standard output, even if it is the console
* `--[no-]sparse`:
enable / disable sparse FS support,
to make files with many zeroes smaller on disk.
Creating sparse files may save disk space and speed up decompression by
reducing the amount of disk I/O.
default: enabled when output is into a file,
and disabled when output is stdout.
This setting overrides default and can force sparse mode over stdout.
* `--rm`:
remove source file(s) after successful compression or decompression
* `-k`, `--keep`:
keep source file(s) after successful compression or decompression.
This is the default behavior.
* `-r`:
operate recursively on dictionaries
* `--format=FORMAT`:
compress and decompress in other formats. If compiled with
support, zstd can compress to or decompress from other compression algorithm
formats. Possibly available options are `gzip`, `xz`, `lzma`, and `lz4`.
* `-h`/`-H`, `--help`:
display help/long help and exit
* `-V`, `--version`:
display version number and exit.
Advanced : `-vV` also displays supported formats.
`-vvV` also displays POSIX support.
* `-v`:
verbose mode
* `-q`, `--quiet`:
suppress warnings, interactivity, and notifications.
specify twice to suppress errors too.
* `-C`, `--[no-]check`:
add integrity check computed from uncompressed data (default: enabled)
* `--`:
All arguments after `--` are treated as files
DICTIONARY BUILDER
------------------
`zstd` offers _dictionary_ compression,
which greatly improves efficiency on small files and messages.
It's possible to train `zstd` with a set of samples,
the result of which is saved into a file called a `dictionary`.
Then during compression and decompression, reference the same dictionary,
using command `-D dictionaryFileName`.
Compression of small files similar to the sample set will be greatly improved.
* `--train FILEs`:
Use FILEs as training set to create a dictionary.
The training set should contain a lot of small files (> 100),
and weight typically 100x the target dictionary size
(for example, 10 MB for a 100 KB dictionary).
Supports multithreading if `zstd` is compiled with threading support.
Additional parameters can be specified with `--train-cover`.
The legacy dictionary builder can be accessed with `--train-legacy`.
Equivalent to `--train-cover=d=8,steps=4`.
* `-o file`:
Dictionary saved into `file` (default name: dictionary).
* `--maxdict=#`:
Limit dictionary to specified size (default: 112640).
* `-#`:
Use `#` compression level during training (optional).
Will generate statistics more tuned for selected compression level,
resulting in a _small_ compression ratio improvement for this level.
* `-B#`:
Split input files in blocks of size # (default: no split)
* `--dictID=#`:
A dictionary ID is a locally unique ID that a decoder can use to verify it is
using the right dictionary.
By default, zstd will create a 4-bytes random number ID.
It's possible to give a precise number instead.
Short numbers have an advantage : an ID < 256 will only need 1 byte in the
compressed frame header, and an ID < 65536 will only need 2 bytes.
This compares favorably to 4 bytes default.
However, it's up to the dictionary manager to not assign twice the same ID to
2 different dictionaries.
* `--train-cover[=k#,d=#,steps=#]`:
Select parameters for the default dictionary builder algorithm named cover.
If _d_ is not specified, then it tries _d_ = 6 and _d_ = 8.
If _k_ is not specified, then it tries _steps_ values in the range [50, 2000].
If _steps_ is not specified, then the default value of 40 is used.
Requires that _d_ <= _k_.
Selects segments of size _k_ with highest score to put in the dictionary.
The score of a segment is computed by the sum of the frequencies of all the
subsegments of size _d_.
Generally _d_ should be in the range [6, 8], occasionally up to 16, but the
algorithm will run faster with d <= _8_.
Good values for _k_ vary widely based on the input data, but a safe range is
[2 * _d_, 2000].
Supports multithreading if `zstd` is compiled with threading support.
Examples:
`zstd --train-cover FILEs`
`zstd --train-cover=k=50,d=8 FILEs`
`zstd --train-cover=d=8,steps=500 FILEs`
`zstd --train-cover=k=50 FILEs`
* `--train-legacy[=selectivity=#]`:
Use legacy dictionary builder algorithm with the given dictionary
_selectivity_ (default: 9).
The smaller the _selectivity_ value, the denser the dictionary,
improving its efficiency but reducing its possible maximum size.
`--train-legacy=s=#` is also accepted.
Examples:
`zstd --train-legacy FILEs`
`zstd --train-legacy=selectivity=8 FILEs`
BENCHMARK
---------
* `-b#`:
benchmark file(s) using compression level #
* `-e#`:
benchmark file(s) using multiple compression levels, from `-b#` to `-e#` (inclusive)
* `-i#`:
minimum evaluation time, in seconds (default: 3s), benchmark mode only
* `-B#`, `--block-size=#`:
cut file(s) into independent blocks of size # (default: no block)
* `--priority=rt`:
set process priority to real-time
**Output Format:** CompressionLevel#Filename : IntputSize -> OutputSize (CompressionRatio), CompressionSpeed, DecompressionSpeed
**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.
ADVANCED COMPRESSION OPTIONS
----------------------------
### --zstd[=options]:
`zstd` provides 22 predefined compression levels.
The selected or default predefined compression level can be changed with
advanced compression options.
The _options_ are provided as a comma-separated list.
You may specify only the options you want to change and the rest will be
taken from the selected or default compression level.
The list of available _options_:
- `strategy`=_strat_, `strat`=_strat_:
Specify a strategy used by a match finder.
There are 8 strategies numbered from 1 to 8, from faster to stronger:
1=ZSTD\_fast, 2=ZSTD\_dfast, 3=ZSTD\_greedy, 4=ZSTD\_lazy,
5=ZSTD\_lazy2, 6=ZSTD\_btlazy2, 7=ZSTD\_btopt, 8=ZSTD\_btultra.
- `windowLog`=_wlog_, `wlog`=_wlog_:
Specify the maximum number of bits for a match distance.
The higher number of increases the chance to find a match which usually
improves compression ratio.
It also increases memory requirements for the compressor and decompressor.
The minimum _wlog_ is 10 (1 KiB) and the maximum is 30 (1 GiB) on 32-bit
platforms and 31 (2 GiB) on 64-bit platforms.
Note: If `windowLog` is set to larger than 27, `--long=windowLog` or
`--memory=windowSize` needs to be passed to the decompressor.
- `hashLog`=_hlog_, `hlog`=_hlog_:
Specify the maximum number of bits for a hash table.
Bigger hash tables cause less collisions which usually makes compression
faster, but requires more memory during compression.
The minimum _hlog_ is 6 (64 B) and the maximum is 26 (128 MiB).
- `chainLog`=_clog_, `clog`=_clog_:
Specify the maximum number of bits for a hash chain or a binary tree.
Higher numbers of bits increases the chance to find a match which usually
improves compression ratio.
It also slows down compression speed and increases memory requirements for
compression.
This option is ignored for the ZSTD_fast strategy.
The minimum _clog_ is 6 (64 B) and the maximum is 28 (256 MiB).
- `searchLog`=_slog_, `slog`=_slog_:
Specify the maximum number of searches in a hash chain or a binary tree
using logarithmic scale.
More searches increases the chance to find a match which usually increases
compression ratio but decreases compression speed.
The minimum _slog_ is 1 and the maximum is 26.
- `searchLength`=_slen_, `slen`=_slen_:
Specify the minimum searched length of a match in a hash table.
Larger search lengths usually decrease compression ratio but improve
decompression speed.
The minimum _slen_ is 3 and the maximum is 7.
- `targetLen`=_tlen_, `tlen`=_tlen_:
The impact of this field vary depending on selected strategy.
For ZSTD\_btopt and ZSTD\_btultra, it specifies the minimum match length
that causes match finder to stop searching for better matches.
A larger `targetLen` usually improves compression ratio
but decreases compression speed.
For ZSTD\_fast, it specifies
the amount of data skipped between match sampling.
Impact is reversed : a larger `targetLen` increases compression speed
but decreases compression ratio.
For all other strategies, this field has no impact.
The minimum _tlen_ is 1 and the maximum is 999.
- `overlapLog`=_ovlog_, `ovlog`=_ovlog_:
Determine `overlapSize`, amount of data reloaded from previous job.
This parameter is only available when multithreading is enabled.
Reloading more data improves compression ratio, but decreases speed.
The minimum _ovlog_ is 0, and the maximum is 9.
0 means "no overlap", hence completely independent jobs.
9 means "full overlap", meaning up to `windowSize` is reloaded from previous job.
Reducing _ovlog_ by 1 reduces the amount of reload by a factor 2.
Default _ovlog_ is 6, which means "reload `windowSize / 8`".
Exception : the maximum compression level (22) has a default _ovlog_ of 9.
- `ldmHashLog`=_ldmhlog_, `ldmhlog`=_ldmhlog_:
Specify the maximum size for a hash table used for long distance matching.
This option is ignored unless long distance matching is enabled.
Bigger hash tables usually improve compression ratio at the expense of more
memory during compression and a decrease in compression speed.
The minimum _ldmhlog_ is 6 and the maximum is 26 (default: 20).
- `ldmSearchLength`=_ldmslen_, `ldmslen`=_ldmslen_:
Specify the minimum searched length of a match for long distance matching.
This option is ignored unless long distance matching is enabled.
Larger/very small values usually decrease compression ratio.
The minumum _ldmslen_ is 4 and the maximum is 4096 (default: 64).
- `ldmBucketSizeLog`=_ldmblog_, `ldmblog`=_ldmblog_:
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 _ldmblog_ is 0 and the maximum is 8 (default: 3).
- `ldmHashEveryLog`=_ldmhevery_, `ldmhevery`=_ldmhevery_:
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 - ldmhlog`.
### -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.
### Example
The following parameters sets advanced compression options to those of
predefined level 19 for files bigger than 256 KB:
`--zstd`=windowLog=23,chainLog=23,hashLog=22,searchLog=6,searchLength=3,targetLength=48,strategy=6
BUGS
----
Report bugs at: https://github.com/facebook/zstd/issues
AUTHOR
------
Yann Collet
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