From: Nick Terrell Date: Thu, 6 May 2021 02:44:24 +0000 (-0700) Subject: [zdict] Add a FAQ to the top of zdict.h X-Git-Tag: v1.5.0^2~25^2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1874f0844df9bc95c66e6d461162c9294af1dbbd;p=thirdparty%2Fzstd.git [zdict] Add a FAQ to the top of zdict.h The FAQ covers the questions asked in Issue #2566. It first covers why you would want to use a dictionary, then what a dictionary is, and finally it tells you how to train a dictionary, and clarifies some of the parameters. There is definitely more that could be said about some of the advanced trainers, but this should be a good start. --- diff --git a/lib/zdict.h b/lib/zdict.h index 4b8589f2f..75b05dbf4 100644 --- a/lib/zdict.h +++ b/lib/zdict.h @@ -36,6 +36,145 @@ extern "C" { # define ZDICTLIB_API ZDICTLIB_VISIBILITY #endif +/******************************************************************************* + * Zstd dictionary builder + * + * FAQ + * === + * Why should I use a dictionary? + * ------------------------------ + * + * Zstd can use dictionaries to improve compression ratio of small data. + * Traditionally small files don't compress well because there is very little + * repetion in a single sample, since it is small. But, if you are compressing + * many similar files, like a bunch of JSON records that share the same + * structure, you can train a dictionary on ahead of time on some samples of + * these files. Then, zstd can use the dictionary to find repetitions that are + * present across samples. This can vastly improve compression ratio. + * + * When is a dictionary useful? + * ---------------------------- + * + * Dictionaries are useful when compressing many small files that are similar. + * The larger a file is, the less benefit a dictionary will have. Generally, + * we don't expect dictionary compression to be effective past 100KB. And the + * smaller a file is, the more we would expect the dictionary to help. + * + * How do I use a dictionary? + * -------------------------- + * + * Simply pass the dictionary to the zstd compressor with + * `ZSTD_CCtx_loadDictionary()`. The same dictionary must then be passed to + * the decompressor, using `ZSTD_DCtx_loadDictionary()`. There are other + * more advanced functions that allow selecting some options, see zstd.h for + * complete documentation. + * + * What is a zstd dictionary? + * -------------------------- + * + * A zstd dictionary has two pieces: Its header, and its content. The header + * contains a magic number, the dictionary ID, and entropy tables. These + * entropy tables allow zstd to save on header costs in the compressed file, + * which really matters for small data. The content is just bytes, which are + * repeated content that is common across many samples. + * + * What is a raw content dictionary? + * --------------------------------- + * + * A raw content dictionary is just bytes. It doesn't have a zstd dictionary + * header, a dictionary ID, or entropy tables. Any buffer is a valid raw + * content dictionary. + * + * How do I train a dictionary? + * ---------------------------- + * + * Gather samples from your use case. These samples should be similar to each + * other. If you have several use cases, you could try to train one dictionary + * per use case. + * + * Pass those samples to `ZDICT_trainFromBuffer()` and that will train your + * dictionary. There are a few advanced versions of this function, but this + * is a great starting point. If you want to further tune your dictionary + * you could try `ZDICT_optimizeTrainFromBuffer_cover()`. If that is too slow + * you can try `ZDICT_optimizeTrainFromBuffer_fastCover()`. + * + * If the dictionary training function fails, that is likely because you + * either passed too few samples, or a dictionary would not be effective + * for your data. Look at the messages that the dictionary trainer printed, + * if it doesn't say too few samples, then a dictionary would not be effective. + * + * How large should my dictionary be? + * ---------------------------------- + * + * A reasonable dictionary size, the `dictBufferCapacity`, is about 100KB. + * The zstd CLI defaults to a 110KB dictionary. You likely don't need a + * dictionary larger than that. But, most use cases can get away with a + * smaller dictionary. The advanced dictionary builders can automatically + * shrink the dictionary for you, and select a the smallest size that + * doesn't hurt compression ratio too much. See the `shrinkDict` parameter. + * A smaller dictionary can save memory, and potentially speed up + * compression. + * + * How many samples should I provide to the dictionary builder? + * ------------------------------------------------------------ + * + * We generally recommend passing ~100x the size of the dictionary + * in samples. A few thousand should suffice. Having too few samples + * can hurt the dictionaries effectiveness. Having more samples will + * only improve the dictionaries effectiveness. But having too many + * samples can slow down the dictionary builder. + * + * How do I determine if a dictionary will be effective? + * ----------------------------------------------------- + * + * Simply train a dictionary and try it out. You can use zstd's built in + * benchmarking tool to test the dictionary effectiveness. + * + * # Benchmark levels 1-3 without a dictionary + * zstd -b1e3 -r /path/to/my/files + * # Benchmark levels 1-3 with a dictioanry + * zstd -b1e3 -r /path/to/my/files -D /path/to/my/dictionary + * + * When should I retrain a dictionary? + * ----------------------------------- + * + * You should retrain a dictionary when its effectiveness drops. Dictionary + * effectiveness drops as the data you are compressing changes. Generally, we do + * expect dictionaries to "decay" over time, as your data changes, but the rate + * at which they decay depends on your use case. Internally, we regularly + * retrain dictionaries, and if the new dictionary performs significantly + * better than the old dictionary, we will ship the new dictionary. + * + * I have a raw content dictionary, how do I turn it into a zstd dictionary? + * ------------------------------------------------------------------------- + * + * If you have a raw content dictionary, e.g. by manually constructing it, or + * using a third-party dictionary builder, you can turn it into a zstd + * dictionary by using `ZDICT_finalizeDictionary()`. You'll also have to + * provide some samples of the data. It will add the zstd header to the + * raw content, which contains a dictionary ID and entropy tables, which + * will improve compression ratio, and allow zstd to write the dictionary ID + * into the frame, if you so choose. + * + * Do I have to use zstd's dictionary builder? + * ------------------------------------------- + * + * No! You can construct dictionary content however you please, it is just + * bytes. It will always be valid as a raw content dictionary. If you want + * a zstd dictionary, which can improve compression ratio, use + * `ZDICT_finalizeDictionary()`. + * + * What is the attack surface of a zstd dictionary? + * ------------------------------------------------ + * + * Zstd is heavily fuzz tested, including loading fuzzed dictionaries, so + * zstd should never crash, or access out-of-bounds memory no matter what + * the dictionary is. However, if an attacker can control the dictionary + * during decompression, they can cause zstd to generate arbitrary bytes, + * just like if they controlled the compressed data. + * + ******************************************************************************/ + /*! ZDICT_trainFromBuffer(): * Train a dictionary from an array of samples. @@ -64,7 +203,14 @@ ZDICTLIB_API size_t ZDICT_trainFromBuffer(void* dictBuffer, size_t dictBufferCap typedef struct { int compressionLevel; /*< optimize for a specific zstd compression level; 0 means default */ unsigned notificationLevel; /*< Write log to stderr; 0 = none (default); 1 = errors; 2 = progression; 3 = details; 4 = debug; */ - unsigned dictID; /*< force dictID value; 0 means auto mode (32-bits random value) */ + unsigned dictID; /*< force dictID value; 0 means auto mode (32-bits random value) + * NOTE: The zstd format reserves some dictionary IDs for future use. + * You may use them in private settings, but be warned that they + * may be used by zstd in a public dictionary registry in the future. + * These dictionary IDs are: + * - low range : <= 32767 + * - high range : >= (2^31) + */ } ZDICT_params_t; /*! ZDICT_finalizeDictionary():