1# ZSTD-On-ZFS Library Manual 2 3## Introduction 4 5This subtree contains the ZSTD library used in ZFS. It is heavily cut-down by 6dropping any unneeded files, and combined into a single file, but otherwise is 7intentionally unmodified. Please do not alter the file containing the zstd 8library, besides upgrading to a newer ZSTD release. 9 10Tree structure: 11 12* `zfs_zstd.c` is the actual `zzstd` kernel module. 13* `lib/` contains the unmodified version of the `Zstandard` library 14* `zstd-in.c` is our template file for generating the single-file library 15* `include/`: This directory contains supplemental includes for platform 16 compatibility, which are not expected to be used by ZFS elsewhere in the 17 future. Thus we keep them private to ZSTD. 18 19## Updating ZSTD 20 21To update ZSTD the following steps need to be taken: 22 231. Grab the latest release of [ZSTD](https://github.com/facebook/zstd/releases). 242. Copy the files output by the following script to `module/zstd/lib/`: 25`grep include [path to zstd]/contrib/single_file_libs/zstd-in.c | awk '{ print $2 }'` 263. Remove debug.c, threading.c, and zstdmt_compress.c. 274. Update Makefiles with resulting file lists. 28 29~~~ 30 31Note: if the zstd library for zfs is updated to a newer version, 32the macro list in include/zstd_compat_wrapper.h usually needs to be updated. 33this can be done with some hand crafting of the output of the following 34script (on the object file generated from the "single-file library" script in zstd's 35contrib/single_file_libs): 36`nm zstd.o | awk '{print "#define "$3 " zfs_" $3}' > macrotable` 37 38 39## Altering ZSTD and breaking changes 40 41If ZSTD made changes that break compatibility or you need to make breaking 42changes to the way we handle ZSTD, it is required to maintain backwards 43compatibility. 44 45We already save the ZSTD version number within the block header to be used 46to add future compatibility checks and/or fixes. However, currently it is 47not actually used in such a way. 48