(NSS) A major Multics implementation project during the 1970s. The initial Multics file system design had evolved from the one-huge-disk world of CTSS. When multiple disk units were used they were just assigned increasing ranges of disk addresses, so a segment could have pages scattered over all disks on the system. This provided good I/O parallelism but made crash recovery expensive. NSS redesigned the lower levels of the file system, introducing the concepts of logical volume and physical volume and a mapping from a Multics directory branch to a VTOC entry for each file. The new system had much better recovery performance in exchange for a small space and performance cost.