Dedup isn't quite the same as sparse files. If the filesystem is unaware of dedup, then it still has to allocate its own structures corresponding to the dedup'

Dedup isn't quite the same as sparse files. If the filesystem is unaware of dedup, then it still has to allocate its own structures corresponding to the dedup'ed space, seeking to the next hole or next allocated block won't work (not that many applications are smart about holes or that MS didn't already suck in that area), etc. Dedup - even in its most absolutely simple form of just detecting zero blocks - does at least avoid the fatal problem of allocating actual disk space uselessly, but sparse files are still basically a filesystem problem and need to be treated as such.