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Comment by sprayk | Hacker News Reader

This was what I was left wondering as well. rsync for sure does not have any FS-specific code that I can find. If it uses the FS APIs provided by the OS, just like every other program that touches files, why would we expect anything other than boring old rsync doing it's thing?

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rsync has a ton of HFS+ specific code. Making a faithful duplicate of a file (specifically its metadata) is a feat on its own: http://blog.plasticsfuture.org/2006/03/05/the-state-of-backu...

Upstream rsync does not seem to have any special-casing for mac/hfs. I'd wager Apple's extended-attribute handling is added in this patch[0] for the version distributed with macOS.

Even here, there isn't special casing for HFS. Instead, a special library function, copyfile()[1], is used to handle copying files and their associated metadata.

It seems this function was introduced in Mac OS X 10.5, which was after the article you linked. I'd wager copyfile() was introduced in response to the unwieldy file copy mechanics.

After discovering how copyfile() is used in rsync, I am fairly confident that rsync works so well on the new FS as a result of Apple implementing a fairly solid copyfile() for APFS.

[0] https://opensource.apple.com/source/rsync/rsync-20/patches/E... [1] http://www.manpagez.com/man/3/copyfile/

You're looking in the wrong place. Install the homebrew version, it has 3 patches for macOS that aren't in the upstream.

https://github.com/Homebrew/homebrew-core/blob/master/Formul...

The Mac version has some file system specific code https://developer.apple.com/legacy/library/documentation/Dar...:

”E, --extended-attributes copy extended attributes, resource forks”

Filenames are treated a bit differently on APFS. Not sure what else, but it seems there's a lot of other bizarre file metadata on OS X that you might want to sync.

edit: not really sure though, because it doesn't seem like this program cares about any of those metadata.

It cares about those metadata a lot. Backups wont be bootable without it. And there is also disastrous data loss without it, a lot of files have their actual file data embedded in the metadata (resource forks et al). It's nuts because it's based on a 30 year old filesystem. That's one of the reasons why the smooth transition is impressive.

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