Physical Layout: The On-Disk Structure
At the disk level, ODS represents the filesystem as an array of blocks, a block being 512 contiguous bytes on one physical disk (volume). Disk blocks are assigned in clusters (originally 3 contiguous blocks but later increased with larger disk sizes). A file on the disk will ideally be entirely contiguous, i.e. the blocks which contain the file will be sequential, but disk fragmentation will sometimes require the file to located in discontiguous clusters in which case the fragments are called 'extents'. Disks may be combined with other disks to form a volume set and files stored anywhere across that set of disks, but larger disk sizes have reduced the use of volume sets because management of a single physical disk is simpler.
Every file on a Files-11 disk (or volume set) has a unique file identification (FID), composed of three numbers: the file number (NUM), the file sequence number (SEQ), and the relative volume number (RVN). The NUM indicates where in the INDEXF.SYS file (see below) the metadata for the file is located; the SEQ is a generation number which incremented when the file is deleted and another file is created reusing the same INDEXF.SYS entry (so any dangling references to the old file do not accidentally point to the new one); and the RVN indicates the volume number on which the file is stored when using a volume set.
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