The generic (naive) implementation of posix_fallocate cannot provide the
standard mandated guarantee that overwrites would never fail due to the lack
of free space. The fundamental reason is the copy-on-write architecture
of ZFS. Other features like compression and deduplication can also
increase the size difference between the (pre-)allocated dummy content
and the future content.
So, until ZFS can properly implement the feature it's better to report
that it is unsupported rather than providing an ersatz implementation.
Please note that EINVAL is used to report that the underlying file system
does not support the operation (POSIX.1-2008).
illumos and ZoL seem to do the same.
MFC after: 3 weeks
Sponsored by: Panzura
7f26516bbc