From: Yann Collet Date: Fri, 8 May 2020 17:51:37 +0000 (-0700) Subject: updated documentation regarding ZSTD_CLEVEL X-Git-Tag: v1.4.5^2~27^2~2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e873697625fa2a9f9a31c1851ebd15107b62c207;p=thirdparty%2Fzstd.git updated documentation regarding ZSTD_CLEVEL make it clearer that it's useful for `tar --zstd`. --- diff --git a/programs/README.md b/programs/README.md index 7668d49a2..a097cdac7 100644 --- a/programs/README.md +++ b/programs/README.md @@ -10,7 +10,7 @@ There are however other Makefile targets that create different variations of CLI - `zstd-decompress` : version of CLI which can only decompress zstd format -#### Compilation variables +### Compilation variables `zstd` scope can be altered by modifying the following `make` variables : - __HAVE_THREAD__ : multithreading is automatically enabled when `pthread` is detected. @@ -69,11 +69,11 @@ There are however other Makefile targets that create different variations of CLI Example : `make zstd BACKTRACE=1` -#### Aggregation of parameters +### Aggregation of parameters CLI supports aggregation of parameters i.e. `-b1`, `-e18`, and `-i1` can be joined into `-b1e18i1`. -#### Symlink shortcuts +### Symlink shortcuts It's possible to invoke `zstd` through a symlink. When the name of the symlink has a specific value, it triggers an associated behavior. - `zstdmt` : compress using all cores available on local system. @@ -86,7 +86,7 @@ When the name of the symlink has a specific value, it triggers an associated beh - `ungz`, `unxz` and `unlzma` will do the same, and will also remove source file by default (use `--keep` to preserve). -#### Dictionary builder in Command Line Interface +### Dictionary builder in Command Line Interface Zstd offers a training mode, which can be used to tune the algorithm for a selected type of data, by providing it with a few samples. The result of the training is stored in a file selected with the `-o` option (default name is `dictionary`), @@ -106,7 +106,7 @@ Usage of the dictionary builder and created dictionaries with CLI: 3. Decompress with the dictionary: `zstd --decompress FILE.zst -D dictionaryName` -#### Benchmark in Command Line Interface +### Benchmark in Command Line Interface CLI includes in-memory compression benchmark module for zstd. The benchmark is conducted using given filenames. The files are read into memory and joined together. It makes benchmark more precise as it eliminates I/O overhead. @@ -118,7 +118,7 @@ One can select compression levels starting from `-b` and ending with `-e`. The `-i` parameter selects minimal time used for each of tested levels. -#### Usage of Command Line Interface +### Usage of Command Line Interface The full list of options can be obtained with `-h` or `-H` parameter: ``` Usage : @@ -172,16 +172,19 @@ Benchmark arguments : --priority=rt : set process priority to real-time ``` -#### Restricted usage of Environment Variables -Using environment variables to set parameters has security implications. -Therefore, this avenue is intentionally restricted. -Only `ZSTD_CLEVEL` is supported currently, for setting compression level. -`ZSTD_CLEVEL` can be used to set the level between 1 and 19 (the "normal" range). -If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message. -`ZSTD_CLEVEL` just replaces the default compression level (`3`). -It can be overridden by corresponding command line arguments. +### Passing parameters through Environment Variables +`ZSTD_CLEVEL` can be used to modify the default compression level of `zstd` +(usually set to `3`) to another value between 1 and 19 (the "normal" range). +This can be useful when `zstd` CLI is invoked in a way that doesn't allow passing arguments. +One such scenario is `tar --zstd`. +As `ZSTD_CLEVEL` only replaces the default compression level, +it can then be overridden by corresponding command line arguments. -#### Long distance matching mode +There is no "generic" way to pass "any kind of parameter" to `zstd` in a pass-through manner. +Using environment variables for this purpose has security implications. +Therefore, this avenue is intentionally restricted and only supports `ZSTD_CLEVEL`. + +### Long distance matching mode The long distance matching mode, enabled with `--long`, is designed to improve the compression ratio for files with long matches at a large distance (up to the maximum window size, `128 MiB`) while still maintaining compression speed. @@ -233,7 +236,7 @@ The below table illustrates this on the [Silesia compression corpus]. | `zstd -10 --long`| `3.566` | `16.2 MB/s` | `415.7 MB/s` | -#### zstdgrep +### zstdgrep `zstdgrep` is a utility which makes it possible to `grep` directly a `.zst` compressed file. It's used the same way as normal `grep`, for example :