Packet writing allows the smallest increment to be written to a disk.
A packet always has a 7 block overhead, 4 blocks run-in, 2 blocks for run-out and a link block. The packet can then be a fixed size, or a variable size. This compares to a track at once where the minimum track length is 300 blocks, with an extra 150 blocks for run-in, run-out and linking. Thus, there are virtually no spaces between sessions.