Well, in recognition that there might be more than one Linux filesystem with extents and so that reflinks might be more generally useful, the extent-same ioctl got lifted up to be in the VFS layer of the kernel instead of just in btrfs.
And the good news is that XFS recently became able to make use of it.
I’m on the btrfs mailing list and there are way too many posts regarding filesystems that give ENOSPC and become unavailable for writes, or systems that were unexpectedly powered off and when powered back on the btrfs filesystem is completely lost.
For me the issue has been that when the filesystem has been near-full (around 5% or lower), btrfs has silently just created files full of zero-bytes. The checksums in git-annex have saved me from losing my precious family photos.
ZFS is great. It doesn’t have out-of-band deduplication or reflinks though, and they don’t plan to any time soon.
Quoting the US President-elect: “Sad!”