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Comment by drdexebtjl | Hacker News Reader

Hardlinks are the wrong abstraction. You modify one copy and it changes on all linked paths. You want reflinks instead.

edit: spellchecker corrected reflinks, this is what I meant.

The difference here is that git objects don't change. When you do a `git gc`, you do get new objects at new inodes, but they don't get written to the existing ones. Try this out and you'll see local git clones do indeed create hardlinks:

``` cd $(mktemp -d) git init test ( cd test; touch README.md; git add README.md; git commit -m "Test"; ) git clone test/.git test_clone find test_clone/.git/objects test/.git/objects -type f -printf "%i\t%p\n" | sort ```

You'll see the same inodes used in both clones.

Yeah, but this only works for the immutable object store. The rest of the tree is still copied.

CoW and reflinks let you share everything, including the non-immutable paths, and even stuff like node_modules.

Not relevant to comparing clones and worktrees, both create a fresh working copy from the repository data, and object store files are not modified in place (or at all really).

I believe you're wrong here. If you modify a hardlinked file, the FS will create new inodes under the hood, leaving existing ones intact. So it's like CoW. At least on ext4.

// Didn't double-check, but I've held this belief for over decade now, would be surprised to be proven wrong

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.

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.

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.

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