200 lines
10 KiB
Markdown
200 lines
10 KiB
Markdown
Command Line Interface for Zstandard library
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============================================
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Command Line Interface (CLI) can be created using the `make` command without any additional parameters.
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There are however other Makefile targets that create different variations of CLI:
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- `zstd` : default CLI supporting gzip-like arguments; includes dictionary builder, benchmark, and support for decompression of legacy zstd formats
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- `zstd_nolegacy` : Same as `zstd` but without support for legacy zstd formats
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- `zstd-small` : CLI optimized for minimal size; no dictionary builder, no benchmark, and no support for legacy zstd formats
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- `zstd-compress` : version of CLI which can only compress into zstd format
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- `zstd-decompress` : version of CLI which can only decompress zstd format
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#### Compilation variables
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`zstd` scope can be altered by modifying the following compilation variables :
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- __HAVE_THREAD__ : multithreading is automatically enabled when `pthread` is detected.
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It's possible to disable multithread support, by setting HAVE_THREAD=0 .
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Example : make zstd HAVE_THREAD=0
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It's also possible to force compilation with multithread support, using HAVE_THREAD=1.
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In which case, linking stage will fail if `pthread` library cannot be found.
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This might be useful to prevent silent feature disabling.
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- __HAVE_ZLIB__ : `zstd` can compress and decompress files in `.gz` format.
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This is ordered through command `--format=gzip`.
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Alternatively, symlinks named `gzip` or `gunzip` will mimic intended behavior.
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`.gz` support is automatically enabled when `zlib` library is detected at build time.
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It's possible to disable `.gz` support, by setting HAVE_ZLIB=0.
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Example : make zstd HAVE_ZLIB=0
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It's also possible to force compilation with zlib support, using HAVE_ZLIB=1.
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In which case, linking stage will fail if `zlib` library cannot be found.
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This might be useful to prevent silent feature disabling.
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- __HAVE_LZMA__ : `zstd` can compress and decompress files in `.xz` and `.lzma` formats.
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This is ordered through commands `--format=xz` and `--format=lzma` respectively.
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Alternatively, symlinks named `xz`, `unxz`, `lzma`, or `unlzma` will mimic intended behavior.
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`.xz` and `.lzma` support is automatically enabled when `lzma` library is detected at build time.
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It's possible to disable `.xz` and `.lzma` support, by setting HAVE_LZMA=0 .
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Example : make zstd HAVE_LZMA=0
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It's also possible to force compilation with lzma support, using HAVE_LZMA=1.
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In which case, linking stage will fail if `lzma` library cannot be found.
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This might be useful to prevent silent feature disabling.
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- __ZSTD_LEGACY_SUPPORT__ : `zstd` can decompress files compressed by older versions of `zstd`.
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Starting v0.8.0, all versions of `zstd` produce frames compliant with the [specification](../doc/zstd_compression_format.md), and are therefore compatible.
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But older versions (< v0.8.0) produced different, incompatible, frames.
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By default, `zstd` supports decoding legacy formats >= v0.4.0 (`ZSTD_LEGACY_SUPPORT=4`).
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This can be altered by modifying this compilation variable.
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`ZSTD_LEGACY_SUPPORT=1` means "support all formats >= v0.1.0".
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`ZSTD_LEGACY_SUPPORT=2` means "support all formats >= v0.2.0", and so on.
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`ZSTD_LEGACY_SUPPORT=0` means _DO NOT_ support any legacy format.
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if `ZSTD_LEGACY_SUPPORT >= 8`, it's the same as `0`, since there is no legacy format after `7`.
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Note : `zstd` only supports decoding older formats, and cannot generate any legacy format.
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#### Aggregation of parameters
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CLI supports aggregation of parameters i.e. `-b1`, `-e18`, and `-i1` can be joined into `-b1e18i1`.
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#### Dictionary builder in Command Line Interface
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Zstd offers a training mode, which can be used to tune the algorithm for a selected
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type of data, by providing it with a few samples. The result of the training is stored
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in a file selected with the `-o` option (default name is `dictionary`),
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which can be loaded before compression and decompression.
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Using a dictionary, the compression ratio achievable on small data improves dramatically.
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These compression gains are achieved while simultaneously providing faster compression and decompression speeds.
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Dictionary work if there is some correlation in a family of small data (there is no universal dictionary).
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Hence, deploying one dictionary per type of data will provide the greater benefits.
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Dictionary gains are mostly effective in the first few KB. Then, the compression algorithm
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will rely more and more on previously decoded content to compress the rest of the file.
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Usage of the dictionary builder and created dictionaries with CLI:
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1. Create the dictionary : `zstd --train PathToTrainingSet/* -o dictionaryName`
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2. Compress with the dictionary: `zstd FILE -D dictionaryName`
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3. Decompress with the dictionary: `zstd --decompress FILE.zst -D dictionaryName`
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#### Benchmark in Command Line Interface
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CLI includes in-memory compression benchmark module for zstd.
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The benchmark is conducted using given filenames. The files are read into memory and joined together.
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It makes benchmark more precise as it eliminates I/O overhead.
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Multiple filenames can be supplied, as multiple parameters, with wildcards,
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or names of directories can be used as parameters with `-r` option.
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The benchmark measures ratio, compressed size, compression and decompression speed.
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One can select compression levels starting from `-b` and ending with `-e`.
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The `-i` parameter selects minimal time used for each of tested levels.
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#### Usage of Command Line Interface
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The full list of options can be obtained with `-h` or `-H` parameter:
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```
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Usage :
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zstd [args] [FILE(s)] [-o file]
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FILE : a filename
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with no FILE, or when FILE is - , read standard input
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Arguments :
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-# : # compression level (1-19, default:3)
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-d : decompression
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-D file: use `file` as Dictionary
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-o file: result stored into `file` (only if 1 input file)
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-f : overwrite output without prompting and (de)compress links
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--rm : remove source file(s) after successful de/compression
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-k : preserve source file(s) (default)
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-h/-H : display help/long help and exit
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Advanced arguments :
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-V : display Version number and exit
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-v : verbose mode; specify multiple times to increase verbosity
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-q : suppress warnings; specify twice to suppress errors too
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-c : force write to standard output, even if it is the console
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-l : print information about zstd compressed files
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--ultra : enable levels beyond 19, up to 22 (requires more memory)
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--long : enable long distance matching (requires more memory)
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--no-dictID : don't write dictID into header (dictionary compression)
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--[no-]check : integrity check (default:enabled)
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-r : operate recursively on directories
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--format=gzip : compress files to the .gz format
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--format=xz : compress files to the .xz format
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--format=lzma : compress files to the .lzma format
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--test : test compressed file integrity
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--[no-]sparse : sparse mode (default:disabled)
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-M# : Set a memory usage limit for decompression
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-- : All arguments after "--" are treated as files
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Dictionary builder :
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--train ## : create a dictionary from a training set of files
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--train-cover[=k=#,d=#,steps=#] : use the cover algorithm with optional args
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--train-legacy[=s=#] : use the legacy algorithm with selectivity (default: 9)
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-o file : `file` is dictionary name (default: dictionary)
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--maxdict=# : limit dictionary to specified size (default : 112640)
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--dictID=# : force dictionary ID to specified value (default: random)
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Benchmark arguments :
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-b# : benchmark file(s), using # compression level (default : 1)
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-e# : test all compression levels from -bX to # (default: 1)
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-i# : minimum evaluation time in seconds (default : 3s)
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-B# : cut file into independent blocks of size # (default: no block)
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--priority=rt : set process priority to real-time
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```
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#### Long distance matching mode
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The long distance matching mode, enabled with `--long`, is designed to improve
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the compression ratio for files with long matches at a large distance (up to the
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maximum window size, `128 MiB`) while still maintaining compression speed.
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Enabling this mode sets the window size to `128 MiB` and thus increases the memory
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usage for both the compressor and decompressor. Performance in terms of speed is
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dependent on long matches being found. Compression speed may degrade if few long
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matches are found. Decompression speed usually improves when there are many long
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distance matches.
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Below are graphs comparing the compression speed, compression ratio, and
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decompression speed with and without long distance matching on an ideal use
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case: a tar of four versions of clang (versions `3.4.1`, `3.4.2`, `3.5.0`,
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`3.5.1`) with a total size of `244889600 B`. This is an ideal use case as there
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are many long distance matches within the maximum window size of `128 MiB` (each
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version is less than `128 MiB`).
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Compression Speed vs Ratio | Decompression Speed
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---------------------------|---------------------
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| Method | Compression ratio | Compression speed | Decompression speed |
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|:-------|------------------:|-------------------------:|---------------------------:|
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| `zstd -1` | `5.065` | `284.8 MB/s` | `759.3 MB/s` |
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| `zstd -5` | `5.826` | `124.9 MB/s` | `674.0 MB/s` |
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| `zstd -10` | `6.504` | `29.5 MB/s` | `771.3 MB/s` |
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| `zstd -1 --long` | `17.426` | `220.6 MB/s` | `1638.4 MB/s` |
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| `zstd -5 --long` | `19.661` | `165.5 MB/s` | `1530.6 MB/s`|
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| `zstd -10 --long`| `21.949` | `75.6 MB/s` | `1632.6 MB/s`|
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On this file, the compression ratio improves significantly with minimal impact
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on compression speed, and the decompression speed doubles.
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On the other extreme, compressing a file with few long distance matches (such as
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the [Silesia compression corpus]) will likely lead to a deterioration in
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compression speed (for lower levels) with minimal change in compression ratio.
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The below table illustrates this on the [Silesia compression corpus].
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[Silesia compression corpus]: http://sun.aei.polsl.pl/~sdeor/index.php?page=silesia
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| Method | Compression ratio | Compression speed | Decompression speed |
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|:-------|------------------:|-------------------------:|---------------------------:|
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| `zstd -1` | `2.878` | `231.7 MB/s` | `594.4 MB/s` |
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| `zstd -1 --long` | `2.929` | `106.5 MB/s` | `517.9 MB/s` |
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| `zstd -5` | `3.274` | `77.1 MB/s` | `464.2 MB/s` |
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| `zstd -5 --long` | `3.319` | `51.7 MB/s` | `371.9 MB/s` |
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| `zstd -10` | `3.523` | `16.4 MB/s` | `489.2 MB/s` |
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| `zstd -10 --long`| `3.566` | `16.2 MB/s` | `415.7 MB/s` |
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