Does it preserve all metadata, extended attributes, and alternate streams/named forks?
It tries, but there are some things it can't perfectly preserve like the last access time. Instances where it can't duplicate certain types of extended attributes or ownership permissions it will not perform the operation.
https://podcasts.apple.com/podcast/id617416468?i=10006919599...
No word about alternate data streams. I'll pass for now.. Although it's nice to see how much duplicates you have
Q: Does Hyperspace preserve file metadata during reclamation?
A: When Hyperspace replaces a file with a space-saving clone, it attempts to preserve all metadata associated with that file. This includes the creation date, modification date, permissions, ownership, Finder labels, Finder comments, whether or not the file name extension is visible, and even resource forks. If the attempt to preserve any of these piece of metadata fails, then the file is not replaced.
If you find some piece of file metadata that is not preserved, please let us know.
Q: How does Hyperspace handle resource forks?
A: Hyperspace considers the contents of a file’s resource fork to be part of the file’s data. Two files are considered identical only if their data and resource forks are identical to each other.
When a file is replaced by a space-saving clone during reclamation, its resource fork is preserved.