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Brian Behlendorf 13c38c4c45 Use known license string for zzstd 
The Linux kernel MODULE_LICENSE macro only recognizes a handful of
license strings and "BSD" is not one of the them.  Update the macro
to use "Dual BSD/GPL" which is recognized and what the kernel expects
BSD licensed module to use.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10982
Closes #10992
 		2020-10-01 12:22:23 -07:00
..
include Avoid symbol collision with in-kernel zstdlib 2020-08-24 12:20:41 -07:00
lib Import ZStandard v1.4.5 2020-08-20 10:30:06 -07:00
Makefile.in module/zstd: pass -U__BMI__ 2020-08-27 16:07:13 -07:00
README.md Avoid symbol collision with in-kernel zstdlib 2020-08-24 12:20:41 -07:00
zfs_zstd.c Use known license string for zzstd 2020-10-01 12:22:23 -07:00
zstd-in.c Import ZStandard v1.4.5 2020-08-20 10:30:06 -07:00

README.md

ZSTD-On-ZFS Library Manual

Introduction

This subtree contains the ZSTD library used in ZFS. It is heavily cut-down by dropping any unneeded files, and combined into a single file, but otherwise is intentionally unmodified. Please do not alter the file containing the zstd library, besides upgrading to a newer ZSTD release.

Tree structure:

  • zfs_zstd.c is the actual zzstd kernel module.
  • lib/ contains the the unmodified, "amalgamated" version of the Zstandard library, generated from our template file
  • zstd-in.c is our template file for generating the library
  • include/: This directory contains supplemental includes for platform compatibility, which are not expected to be used by ZFS elsewhere in the future. Thus we keep them private to ZSTD.

Updating ZSTD

To update ZSTD the following steps need to be taken:

  1. Grab the latest release of ZSTD.
  2. Update module/zstd/zstd-in.c if required. (see zstd/contrib/single_file_libs/zstd-in.c in the zstd repository)
  3. Generate the "single-file-library" and put it to module/zstd/lib/.
  4. Copy the following files to module/zstd/lib/:
    • zstd/lib/zstd.h
    • zstd/lib/common/zstd_errors.h

This can be done using a few shell commands from inside the zfs repo:

cd PATH/TO/ZFS

url="https://github.com/facebook/zstd"
release="$(curl -s "${url}"/releases/latest | grep -oP '(?<=v)[\d\.]+')"
zstd="/tmp/zstd-${release}/"

wget -O /tmp/zstd.tar.gz \
    "${url}/releases/download/v${release}/zstd-${release}.tar.gz"
tar -C /tmp -xzf /tmp/zstd.tar.gz

cp ${zstd}/lib/zstd.h module/zstd/lib/
cp ${zstd}/lib/zstd_errors.h module/zstd/lib/
${zstd}/contrib/single_file_libs/combine.sh \
    -r ${zstd}/lib -o module/zstd/lib/zstd.c module/zstd/zstd-in.c

Note: if the zstd library for zfs is updated to a newer version, the macro list in include/zstd_compat_wrapper.h usually needs to be updated. this can be done with some hand crafting of the output of the following script: nm zstd.o | awk '{print "#define "$3 " zfs_" $3}' > macrotable

Altering ZSTD and breaking changes

If ZSTD made changes that break compatibility or you need to make breaking changes to the way we handle ZSTD, it is required to maintain backwards compatibility.

We already save the ZSTD version number within the block header to be used to add future compatibility checks and/or fixes. However, currently it is not actually used in such a way.