A Case for NOW (Networks of Workstations) David E. Culler Computer Science Division University of California, Berkeley The natural evolution of the "killer micro" combined with the emergence of the "killer network" is likely to position networks of workstations (NOWs) as the primary computing infrastructure for science and engineering. They will provide a vast pool of computational and storage resources for demanding applications too large for the desktop, as well as the vehicle for interactive computing. In addition to the migration of parallel computing to a mainstream infrastructure, this transformation brings the opportunity to dramatically improve virtual memory and file system performance by using the aggregate DRAM of a NOW as a giant cache for disk and by using collections of workstation disks as a soft RAID. We describe the technical challenges in exploiting these opportunities, including efficient communication hardware and software, global coordination of multiple workstation operating systems, scalable management of metadata, and the tension with commodity system implementations. We discuss progress in the Berkeley NOW project toward a 100-node NOW prototype that demonstrates practical solutions to these challenges.