Does any file system implement Copy on Write mechanism for CP?

From cp man page:

When --reflink[=always] is specified, perform a lightweight copy, where the data blocks are copied only when modified. If this is not possible the copy fails, or if --reflink=auto is specified, fall back to a standard copy.

This works on file systems which support Copy-on-Write (reflink), mainly BTRFS at the moment. XFS reflink implementation is in development ^[1][2].

The keyword to search is reflink. It was recently implemented in XFS.

EDIT: the XFS implementation was initially marked EXPERIMENTAL. This warning was removed in the kernel release 4.16, a number of months after I wrote the above :-).

Linux has a system call that allows userspace processes to tell the kernel to make copy on write copies of files. FICLONERANGE and FICLONE used as options to ioctl allow copy on write copies of files and ranges within files to be made.

This is used by cp --reflink to make the copies where the file system supports this.