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Basic file organization techniques

Given that a file consists, generally speaking, of a collection of records, a key element in file management is the way in which the records themselves are organized inside the file, since this heavily affects system performances ad far as record finding and access. Note carefully that by ``organization'' we refer here to the logical arrangement of the records in the file (their ordering or, more generally, the presence of ``closeness'' relations between them based on their content), and not instead to the physical layout of the file as stored on a storage media, To prevent confusion, the latter is referred to by the expression ``record blocking'', and will be treated later on.

Choosing a file organization is a design decision, hence it must be done having in mind the achievement of good performance with respect to the most likely usage of the file. The criteria usually considered important are:

  1. Fast access to single record or collection of related recors.
  2. Easy record adding/update/removal, without disrupting (1).
  3. Storage efficiency.
  4. Redundance as a warranty against data corruption.
Needless to say, these requirements are in contrast with each other for all but the most trivial situations, and it's the designer job to find a good compromise among them, yielding and adequate solution to the problem at hand. For example, easiness of adding/etc. is not an issue when defining the data organization of a CD-ROM product, whereas fast access is, given the huge amount of data that this media can store. However, as it will become apparent shortly, fast access techniques are based on the use of additional information about the records, which in turn competes with the high volumes of data to be stored.

Logical data organization is indeed the subject of whole shelves of books, in the ``Database'' section of your library. Here we'll briefly address some of the simpler used techniques, mainly because of their relevance to data management from the lower-level (with respect to a database's) point of view of an OS. Five organization models will be considered:




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Next: Pile Up: File Management - II Previous: File Management - II

Franco Callari