+dit(bf(--preallocate)) This tells the receiver to allocate each destination
+file to its eventual size before writing data to the file. Rsync will only use
+the real filesystem-level preallocation support provided by Linux's
+bf(fallocate)(2) system call or Cygwin's bf(posix_fallocate)(3), not the slow
+glibc implementation that writes a zero byte into each block.
+
+Without this option, larger files may not be entirely contiguous on the
+filesystem, but with this option rsync will probably copy more slowly. If the
+destination is not an extent-supporting filesystem (such as ext4, xfs, NTFS,
+etc.), this option may have no positive effect at all.
+