Physical File

Introduction

It refers to the different ways in which file information can be organized, as well as the different ways in which it can be accessed. Since there are 2 levels of vision of the files (physical and logical), we can also talk about 2 aspects of file organization: Organization of logical files and physical files.

The file system is especially concerned with the management of secondary storage space, primarily disk storage.

One way of organizing a file system may be as follows:

File names only need to be unique within a given user directory.

The system name for a given file must be unique for the file system.

In “hierarchical” file systems the system name for a file is usually formed as the “path name” from the root directory to the file.

Logical organization

Contenido

La mayoría de las computadoras organizan los archivos en jerarquías llamadas carpetas, directorios o catálogos . (El concepto es el mismo independientemente de la terminología usada.) Cada carpeta puede contener un número arbitrario") de archivos, y también puede contener otras carpetas. Las otras carpetas pueden contener todavía más archivos y carpetas, y así sucesivamente, construyéndose una estructura en árbol en la que una «carpeta raíz» (el nombre varía de una computadora a otra) puede contener cualquier número de niveles de otras carpetas y archivos. A las carpetas se les puede dar nombre exactamente igual que a los archivos (excepto para la carpeta raíz, que a menudo no tiene nombre). El uso de carpetas hace más fácil organizar los archivos de una manera lógica.

La mayor parte de las estructuras de organizaciones alternativas de archivos se encuentran dentro de estas cinco categorías:.

Batteries

It is the easiest way to organize a file. Data is collected in the order in which it arrives.

Its goal is simply to accumulate a mass of data and store it.

Records can have different or similar fields in a different order. Each field must be self-describing, including both a name and value field. The length of each field must be indicated implicitly with delimiters, explicitly included as another subfield.

Physical File

Introduction

The file system is especially concerned with the management of secondary storage space, primarily disk storage.

One way of organizing a file system may be as follows:

File names only need to be unique within a given user directory.

The system name for a given file must be unique for the file system.

In “hierarchical” file systems the system name for a file is usually formed as the “path name” from the root directory to the file.

Logical organization

Contenido

La mayor parte de las estructuras de organizaciones alternativas de archivos se encuentran dentro de estas cinco categorías:.

Batteries

It is the easiest way to organize a file. Data is collected in the order in which it arrives.

Physical Organization

En la organización física los datos son arreglados por su adyacencia física, es decir, de acuerdo con el dispositivo de almacenamiento secundario. Los registros son de tamaño fijo o de tamaño variable y pueden organizarse de varias formas para constituir archivos físicos.

Se utilizan:.

Magnetic Tape

In this device, the physical file is made up of a set of physical records, and the blocks are organized consecutively, since they are assigned in the same way.

In addition, such records may contain labels that allow greater control over the stored data, and they are the following:

magnetic discs

The physical file on a disk is a collection of physical records of equal size, which can be organized consecutively, linked, or with a mapping table.

To choose a file organization, certain criteria must be taken into account:

If a file is to be processed only in batches, accessing all records each time, then fast access for single record retrieval is of minimal concern. A file stored on CD-ROM will never be updated, so ease of update is not considered. For the storage economy, there should be a minimum data redundancy, this redundancy is the fundamental means to increase the speed of data access.

This type of organization shows, in turn, 2 important aspects: Free Space Allocation Methods and File Storage Space Allocation..

A free space allocation method determines how an Operating System controls the places on the disk that are not being occupied.

To control free space, one of the theoretical methods can be used as a basis: Bit Vector, Linked List, By Grouping and by Counter..

If you have an array of bits, the number of bits it has represents each sector of the disk, that is, if sectors 10 and 11 are occupied, its representation will be:

There is a header in which there is the address of the first empty sector, that sector, in turn, has a pointer to the next block, and so on until a mark is found indicating that there is no longer free space, as shown in the following figure:

It is similar to the linked list, except that in each sector there is a group of pointers to several empty spaces, at the end of each block there is a pointer to another group of pointers, observe the figure.

Here, for each set of contiguous blocks that are empty, there is, for each pointer, a start number and the size of the group of empty sectors.

A disk space allocation method determines how an Operating System controls the disk locations occupied by each data file. Basically the identification of the file, start sector and end sector must be controlled.

To control the space occupied on disk, one of the theoretical methods can be used as a basis: Contiguous Allocation, Linked Allocation, Indexed Allocation..

This method consists of allocating disk space in such a way that the addresses of all blocks corresponding to a file define a linear order. For example:.

In this method, each file is a linked list of disk blocks. In the directory there is a pointer to the start block and a pointer to the end block for each file. In each of the blocks where a file is located there is a pointer to the next block in the list. For example:.

As already seen, linked allocation solves external fragmentation problems, however, linked allocation does not efficiently support direct file access. Indexed allocation solves this problem by putting all pointers in a single location: The index block..

Each file has its index block, which is an array of disk block addresses.