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Sebastian Gottschall 7ce9da0bea do a cyclic seek for unused memory objects in pool 
In non regular use cases allocated memory might stay persistent in memory
pool. This small patch checks every minute if there are old objects which
can be released from memory pool.

Right now with regular use, the pool is checked for old objects on each
allocation attempt from this pool. so basically polling by its use. Now
consider what happens if someone writes a lot of files and stops use of
the volume or even unmounts it. So the code will no longer check if
objects can be released from the pool. Already allocated objects will
still stay in pool cache. this is no big issue for common use. But
someone discovered this issue while doing tests. personally i know this
behavior and I'm aware of it. Its no big issue. just a enhancement

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Closes #10938
Closes #10969
 		2020-10-01 12:22:48 -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 do a cyclic seek for unused memory objects in pool 2020-10-01 12:22:48 -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.