module/zstd/README.md


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38


    Contents⊕
  
  
  ZSTD-On-ZFS Library
  Manual
  
  Introduction
  Updating ZSTD
  Altering ZSTD and
  breaking changes
  
  
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 are the actual zfs kernel
module hooks.
lib/ contains the unmodified version of the
Zstandard library
zstd-in.c is our template file for generating the
single-file 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:

Grab the latest release of ZSTD.
Copy the files output by the following script to
module/zstd/lib/:

grep include [path to zstd]/contrib/single_file_libs/zstd-in.c  | awk '{ print $2 }'
3. Remove debug.c, threading.c, and zstdmt_compress.c. 4. Update
Makefiles with resulting file lists. 5. Follow symbol renaming notes in
include/zstd_compat_wrapper.h
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.