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This is a version of "brkirch"'s afsctool utility that allows end-users to leverage HFS+ compression.
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.kdev4 more timing tweaking, and add an allow-recompression (aka undo) mode … Jan 1, 2019
_cmake/modules rename cmake module dir Dec 28, 2018
src afsctool : fix an erroneous error message Jun 13, 2019
.gitignore Detect HFS and also APFS from the f_fstypename string Mar 31, 2017
CMakeLists.txt Make certain the LZVN header is found Jun 10, 2019
License.txt Initial commit; original v34 version with new CMakeLists.txt file May 27, 2015 README: link to openzfsonosx forum discussion Jan 5, 2019
afsctool_34.kdev4 Added support for parallel compressing in a configurable Jun 3, 2015

This is my version of brkirch's afsctool

AFSC (Apple File System Compression) tool is a utility that can be used to apply HFS+ compression to file(s), decompress HFS+ compressed file(s), or get information about existing HFS+ compressed file(s). Mac OS 10.6 or later is required. See:

I have made several modifications, mostly concerning the compression feature:

  • improved error reporting
  • an attempt to reduce memory pressure compressing large files
  • support for multiple files/folders specified on the commandline
  • a backup option while compressing (that comes in addition to the existing undo if something went wrong)
  • support for files that are read-only (and/or write-only) by changing their permissions temporarily. No error checking is done for this feature; failure will lead to errors that are already caught.

The main new feature that justifies the version bump, however, is the parallel processing feature, allowing the user to specify an arbitray (though positive :)) number of threads that will compress the specified files in parallel. This feature has two modes that each perform best in specific conditions:

  • perform only the actual compression in parallel. Disk IO (reading the file into memory, writing it back to disk) is done with exclusive access to prevent disk trashing. This mode is selected with the -j option.
  • perform all file compression steps in parallel. This mode is suitable for solid- state disks, file collections that reside on different disks or collections with files of different sizes (including notably very large files). This mode is selected with the -J option. The performance difference is never enormous in my testing, but YMMV.

Interestingly, the optimum performance (on large collections) is not necessarily obtained with as many worker threads as there are CPU cores. Collections with a significant number of very large files and many more smaller files can apparently be processed faster when using a rather larger number of workers, with a gain approaching the number of cores.

Version 1.6.9 introduces two new options for the parallel compression mode:

  • size-sorting the item list (-S option). The parallel workers each take the next uncompressed file off the item list, which normally is in the order in which files were found during the scan for compressable files in the user arguments. Sorting this list means that the smallest files will be compressed first, which may be of interest when the target disk is very (almost) full and the list also contains large files (which require more space for the temporary backup).
  • reverse workers (-RM option) when using a sorted list. This option configures out of the worker threads to take items off the back of the list. The main purpose of the option is to combine -Ji -S -Ri, i.e. compress the largest files first. This may be beneficial to help limit (free space) fragmentation (if that's the goal, using a single worker thread is probably best).

Version 1.7 introduces support for other compression types as added by Apple in OS X 10.9 and later. Currently only LZVN support is fully implemented (though decompression of LZFSE should work if the OS supports it). Note that LZVN support requires a headerfile not currently installed by the original author's LZVN repo; use my fork instead and build it with cmake. --> This version also makes the current ZLIB compression mode optional, that uses a compression buffer that is allocated all at once (and is thus almost always too large. The new default mode adopts the approach also used for LZVN compression, where the memory buffer is grown as needed and thus only gets as large as needed (typically 4-5x smaller than in the singleshot mode). Singleshot mode might be marginally faster when enough RAM is available.


afsctool depends on zlib (v1.2.8 or newer) and Google's sparsehash library and on CMake and pkgconfig for building. The OS zlib copy may be recent enough (it is on 10.12 and later) but to be certain to obtain the latest versions of both, use a package manager like MacPorts, Fink or HomeBrew. Be sure to follow the general installation and usage instructions for those projects, in particular how to update your PATH.

using MacPorts:

port install sparsehash zlib cmake pkgconfig

using HomeBrew:

brew install google-sparsehash zlib cmake pkgconfig

(Setting PKG_CONFIG_PATH is only required with HomeBrew.)


With the dependencies installed you can now build afsctool. In a directory of your choice:

git clone git://
mkdir afsctool/build
cd afsctool/build
cmake -Wno-dev ..

This will leave the afsctool executable in afsctool/build; you can move it anywhere you like from there. You can also do an "official" install, to /usr/local/bin by default:

cd afsctool/build
sudo make install/fast V=1 VERBOSE=1

A word about the other executable (zfsctool)

This repository also builds another utility, zfsctool.


This is a stripped-down and adapted version of afsctool, aiming to provide a comparable offline/post-hoc recompression of selected files and folders on ZFS as afsctool does on HFS+ and APFS.

ZFS is organised in datasets (roughly comparable to partitions), and offers true transparent in-filesystem compression. Because of this, one typically uses a reasonably efficient but fast compressor (like lz4). Compression is set at the dataset level, but files are compressed with the codec in used when they were written (and that means they can contain chunks compressed with different compressors). Afsctool-like recompression can thus be obtained by changing the dataset compression property, rewriting all files of interest, and resetting the compression property. Contrary to HFS this will also affect any other files being written during the rewrite but that's life.

Zfsctool is a work in progress that doesn't yet do anything except for printing out statistics. There is not currently an API provided by the ZFS libraries which allow getting or setting the compression property so I use the zfs driver command and parse its output (striving very hard to call the command as little as possible; once per dataset during the file collection phase). As a bonus, zfsctool only has a runtime dependency on ZFS. See

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