On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote:

I guess it's up to filesystem if it wants to reuse the same spot to write
data or not. I think your assumptions works for ext4 and xfs. I wouldn't
be that sure for btrfs or other filesystems with CoW support.

Or XFS with reflinks for that matter.  Which currently can't be
combined with DAX, but I had a somewhat working version a few month
ago.