File Sequence - File Scattering

File Scattering

A file sequence located within a mass storage device is said to be contiguous if:

• every file in the sequence is unfragmented, i.e. each file is stored in one contiguous and ordered piece of storage space (ideally in one or multiple, but contiguous, extents);
• consecutive files in the sequence occupy contiguous portions of storage space (extents, yet consistently with their file ordering).

File contiguity is a more practical requirement for file sequences than just their locality of reference, because it is related to the storage medium hosting the whole sequence than to the sequence itself (or its metadata). At the same time, it is a "high-level" feature, because it is not related to the physical and technical details of mass storage itself: particularly, file contiguity is realized in different ways according to the storage device's architecture and actual filesystem structure. At "low level", each file in a contiguous sequence must be placed in contiguous blocks, in spite of reserved areas or special metadata required by the filesystem (like inodes or inter-sector headers) actually interleaving them.

File contiguity is, in most practical applications, "invisible" at operating-system or user levels, since all the files in a sequence are always available to applications in the same way, regardless of their physical location on the storage device (due to operating systems hiding the filesystem internals to higher-level services). Indeed, file contiguity may be related to I/O performance when the sequence is to be read or written in the shortest time possible. In some contexts (like optical disk burning - also cfr. below), data in a file sequence must be accessed in the same order as the file sequence itself; in other contexts, a "random" access to the sequence may be required. In both cases, most professional filesystems provide faster access strategies to contiguous files than non-contiguous ones. Data pre-allocation is crucial for write access, whereas burst read speeds are achievable only for contiguous data.

When a file sequence is not contiguous, it is said to be scattered, since its files are stored in sparse locations on the storage device. File scattering is the process of allocating (or re-allocating) a file sequence as being (or becoming) uncontiguous. That is often associated with file fragmentation too, where each file is also stored in several, non-contiguous blocks; mechanisms contributing to the former are usually a common cause to the latter too. The act of reducing file scattering by means of allocating (in the first place) or moving (for already-stored data) files in the same sequence near together on the storage medium is called (file) file descattering. A few defragmentation strategies and dedicated software are able to both defragment single files and descatter file sequences.