Optimizing File Availability in a Secure Serverless Distributed File System

Farsite is a secure, scalable, distributed file system that logically functions as a centralized file server but that is physically realized on a set of client desktop computers. Farsite provides security, reliability, and availability by storing replicas of each file on multiple machines. It continuously monitors machine availability and relocates replicas as necessary to maximize the effective availability of the system. We evaluate several replica-placement methods using large-scale simulation with machine availability data from over 50,000 desktop computers. We find that initially placing replicas in an availabilitysensitive fashion yields pathological results, whereas very good results are obtained by random initial placement followed by incremental improvement using a scalable, distributed, fault-tolerant, and attack-resistant hillclimbing algorithm. The algorithm is resilient to severe restrictions on communication and replica placement, and it does not excessively co-locate replicas of different files on the same set of machines.