c01816a97f
This has three advantages over write(2)/read(2): * Fewer context switches and data copies * Mostly preserves a file's sparseness * On some file systems (currently NFS 4.2) the file system will perform the copy in an especially efficient way. Reviewed by: rmacklem MFC after: 2 weeks Sponsored by: Axcient Differential Revision: https://reviews.freebsd.org/D26377