> I'm sure there is also a way of making a hole in an existing file.

I'd love to hear it, as I don't know of nay (short, of course, of
writing directly to the underlying disk device).

>> Hm, which makes me think, that maybe we want SEEK_HOLE, [...]
> That isn't the right place, seek is (probably) handled well above the
> VOP layer.

Probably?  Check out VOP_SEEK.  (But before you get too het up over it,
also check out the comment in sys_pread/sys_pwrite:
        /*
         * XXX This works because no file systems actually
         * XXX take any action on the seek operation.
         */
meaning that you want to be careful what you do to seeks.  Also note
that VOP_SEEK doesn't take a whence argument; that _is_ handled above
the VOP layer.)

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