module/zstd/README.md

   1 # ZSTD-On-ZFS Library Manual
   2
   3 ## Introduction
   4
   5 This subtree contains the ZSTD library used in ZFS. It is heavily cut-down by
   6 dropping any unneeded files, and combined into a single file, but otherwise is
   7 intentionally unmodified. Please do not alter the file containing the zstd
   8 library, besides upgrading to a newer ZSTD release.
   9
  10 Tree structure:
  11
  12 * `zfs_zstd.c` are the actual `zfs` kernel module hooks.
  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
  21 To update ZSTD the following steps need to be taken:
  22
  23 1. Grab the latest release of [ZSTD](https://github.com/facebook/zstd/releases).
  24 2. 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 }'`
  26 3. Remove debug.c, threading.c, and zstdmt_compress.c.
  27 4. Update Makefiles with resulting file lists.
  28 5. Follow symbol renaming notes in `include/zstd_compat_wrapper.h`
  29
  30 ## Altering ZSTD and breaking changes
  31
  32 If ZSTD made changes that break compatibility or you need to make breaking
  33 changes to the way we handle ZSTD, it is required to maintain backwards
  34 compatibility.
  35
  36 We already save the ZSTD version number within the block header to be used
  37 to add future compatibility checks and/or fixes. However, currently it is
  38 not actually used in such a way.