Although most people clean their houses or apartments on a weekly or
monthly basis, an annual spring cleaning is certainly useful. Similarly, networks
sometimes need massive, disruptive cleaning. Cooling systems must be
powered off, drained, cleaned, and refilled. Messy nests of wires become
impediments to working effectively and sometimes must be tidied. Large
volumes of data must be moved between file servers to optimize performance
for users or simply to provide room for growth. Improvements that
involve many changes can be done much more efficiently if all users agree
to a large window of downtime. The flight director technique guides the