Add rsyncable option to gzip and zstd compression #310
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Currently this requires linking a patched or static version of the libraries, where support for "rsyncable" compression has been added. For zstd specifically, this also requires defining a special macro `-DZSTD_STATIC_LINKING_ONLY=1`, before #include <zstd.h> (in flags). Signed-off-by: Anders F Björklund <anders.f.bjorklund@gmail.com>
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Currently this requires linking a patched or static version of the
libraries, where support for "rsyncable" compression has been added.
For zstd specifically, this also requires defining a special macro
-DZSTD_STATIC_LINKING_ONLY=1, before #include <zstd.h> (in flags).Assuming that the static libraries have been installed under /opt:
The original patch for zlib seems lost, but resurrected it from deltarpm
https://svn.uhulinux.hu/packages/dev/zlib/patches/02-rsync.patchFor zstd it is now included by default (since version 1.3.8), but it still a
"experimental parameter" and thus requires static linking like above.
The idea behind rsyncable is to add some "synchronization points",
using a rolling hash instead of a fixed size, to help with delta transfer.
The output format is compatible, but the extra hash makes it slower...
Thus it is not supported for lz4, and not recommended for fast zstd*.
* compression levels below 6 or so, i.e. normally used with 9 or 15
"The rsync algorithm": https://rsync.samba.org/tech_report/
gzip -9 --rsyncableThe pigz implementation uses a faster hash, than the original gzip.
zstd -9 --rsyncable