I brought up NFS because it's an example of a service that implements an abstraction but does so in a way that undermines the assumptions of the implemented abstraction. I do not disagree that local disks are an unrealistic strategy for creating a scalable, fault-tolerant system. The disk abstraction is of limited utility when creating such systems, because "disk thinking" leads to giving in to seductive assumptions about the performance and reliability of the storage resources you have at your disposal.