David Woodhouse wrote:

I was dubious about adding a check for all zeroes before
falling back to zlib, since it'll slow down the common case where we write
non-zero data.

I suggest you bench this first.  As a prototype, just add some code to
touch the data and see how much difference it makes.  Quite possibly
not much - view it as a fancy cache preload.   Plus for real data,
it'll bale out after the first byte...

A full holepunch()/perforate() thing is, as Dave points out, a trickier
version of truncate.  It's quite a hassle for not a lot of gain on
the block filesystems.