A study of application demands from a production datacenter of 1500
servers shows that except for a few outliers, application demands can
be generally met by a network that is slightly
oversubscribed. Eliminating oversubscription is hence a needless
overkill. In a significant departure from recent proposals that do so,
we advocate a hybrid architecture. The base network is
provisioned for the average case, is oversubscribed, and can be built
with any of the existing network designs. To tackle the hotspots that
remain, we add extra links on an on-demand basis. These links
called~flyways provide additional capacity where and when
needed. Our results show that even a few additional flyways
substantially improve performance (by over 50%), as long as they are
added at the right place in the network. We consider two design
alternatives for adding flyways at negligible additional cost: one
that uses wireless links~(60ghz or 802.11n) and another that uses
commodity switches to add capacity in a randomized manner.