You're about to be very surprised!

  echo old > a
  ln a b
  echo new > a  
  cat a b                   # prints "new" and "new"

Some programs like GNU sed [1] do create new inodes, but that is a property of the application, not the filesystem:

  echo old > a
  ln a b
  sed -i 'c new' a
  cat a b                   # prints "new" and "old"

With reflinks in a CoW filesystem the inodes are different from the start, but share the same underlying blocks:

  echo old > a
  cp --reflink=always a b
  ls -i a b                 # prints different inodes
  echo new > a
  cat a b                   # prints "new" and "old"

[1] https://www.gnu.org/software/sed/manual/html_node/Command_00...

Cool, I love surprises :D

Thanks for examples and especially the doc link. I don't normally use hard links, and for some reason was sure that's how they behave.

Maybe my belief indeed stemmed from observing software which overwrites opened file with a new copy.

No this is completely incorrect. The filesystem doesn’t break a link just if you open it. Your editor or higher level utility is free to do so (by unlinking and copying the data into a new file/inode), but this is not how the filesystem works.

Changing a hardlinked file changes it everywhere, unless your specific editor happens to create a new inode. Thus it depends on your editor.

CoW is similar except the new inode is created by the file system itself and is independent of the editor.