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The logical organization of the file system

File Attributes The concept of file includes not only the stored data and the name and attributes. Attributes - that is, information that describes the properties of the file. Examples of possible file attributes: file type (regular file, directory, special file, etc.); the owner of the file; creator of the file; a password for access to the file; Information on the operations allowed access to the file; time of creation, last access and the latest developments; the current size of the file; Maximum file size; o o sign read only; sign hidden file; sign system file; sign archive file; sign binary / character; sign interim (removed after the completion of the process); token lock; the length of an entry in the file; a pointer to the key field in the record; key length. Set the file attributes defined by the specifics of the file system: the file systems of different types to describe the files can be used different sets of attributes. For example, the file systems that support unstructured files, no need to use the last three in the list of attributes associated with structuring the file. In a single operating system in a set of attributes will not have characteristics that are relevant to users and protection, such as file owner, the creator of the file, the password to access the file, information about how to access the file. Users can gain access to the attributes, using funds provided for these purposes, the file system. Usually allowed to read the value of any attribute, and change - just a few. For example, the user can change the file access rights (provided that it has authorized to do so), but change the date of creation or the current size of the file it is not permitted. The values of the attributes of files can be directly found in catalogs, as in the file system, MS-DOS The figure represented an entry in the directory structure containing a mere symbolic name and file attributes. Here, the letters are marked signs of the file: R - read-only, A - Archive H - hidden, S - system. Another option is to place the attributes of special tables when directories contain only links to the tables. This approach is realized, for example, a file system ufs OS UNIX. In this file system directory structure is very simple. Record of each file contains a short character file name and a pointer to inodes file, so named ufs table, which focused the value of the file attributes In either version of the catalog provides a link between file names and the actual files. However, an approach where the file name is separated from its attributes, makes the system more flexible. For example, the file can be easily incorporated in several directories. The record of this file in different directories can contain different simple names, but in the links will contain the same number of inodes. The logical organization of the file In general, the data contained in the file, have a certain logical structure. This structure is the basis for policy, designed to process the data. For example, the text can be properly displayed, the program should be able to isolate specific words, lines, paragraphs, etc. The signs separating one from another building block, can be a certain code sequence or a well-known program offset values of these structural elements relative to the beginning of the file. Maintenance of the data structure can be either entirely the responsibility of the application, or in varying degrees, this work can take on the file system. In the first case, where all actions related to the structuring and interpretation of the contents of the file entirely the responsibility of the application file to the FS sequence of unstructured data. Annex makes requests to the file system on the input-output using common to all applications, system tools, for example, by specifying the offset from the beginning of the file and the number of bytes that need to be considered or write. Received by the application stream of bytes is interpreted in accordance with the framework of the program logic. For example, the compiler generates, and the editor of the links takes quite some object module format of the program. At the same file format, which stores the object module is known only to those programs. Emphasize that the interpretation of the data is not linked with a valid way to store them in the file system. Model file, in which the contents of the file to an unstructured sequence (stream) of bytes became popular along with the operating system UNIX, and now it is widely used in most modern operating systems, including MS-DOS, Windows NT/2000, NetWare. Unstructured model file allows easy sharing of files among multiple applications: different applications may in its own structure and interpret the data contained in the file. Another model is a file that was used in the operating system OS/360, DEC RSX and VMS, and currently is used very often - is a structured file. In this case, the maintenance of the structure of a file is requested the file system. A file system sees a file as an ordered sequence of logical records. An application can apply to the Federal Assembly with requests for input-output-level records, such as take record 25 in the file FILE.DOC. FS should have information about the structure of the file, sufficient to identify any record. FS annex provides access to the records, and all further processing of the data contained in the record, the application. The development of this approach become a database management system (DBMS) that supports not only a complex structure, but also the relationship between them. The logical record is the smallest element of data that can operate a programmer at the exchange with an external device. Even if the physical exchange of the device being more than one operating system to provide the programmer access to a single logical record. A file system can use two ways to access the logical records: read or write logical records sequentially (serial access) or to position the file on record with a specified number (direct access). Obviously, the OS may not support all possible ways to structure the data in the file, so in the operating system, in which there is general support for the logical structuring of files, it is for a small number of widespread schemes logical organization of the file. These include ways to structure the true representation of data in the form of records, the length of which is fixed in the files in this case, access to the n-th recording is carried out either by reading the serial (n-1) prior to the records, either directly at the address, calculated by its serial number . For example, if L - length of the recording, the start address of n-th record is equal to L * n. Note that this logical organization of the size of recording fixed within a file and write in different files that belong to the same file system, may have different sizes. Another way is to structure the data in the form of a sequence of records, which varies in a single file. If the distance between the values of lengths of records as shown in Fig. 7.7, b, to find specific records system should consistently take all previous records. Calculate the address of its record number in such a logical organization of the file is not possible, and therefore can not be applied more efficient method of direct access. The files, access to records which are consistently on the numbers of positions, called neindeksirovannymi or consistent. Another type of files are indexed files, they allow more rapid access to a single logical record. In the indexed file records have one or more key (index) of fields and can be addressed by specifying the values of these fields. For quick retrieval of data in the Index provides a special index table, in which the values of key fields is in line address external memory. This address can point directly to the desired entry, or some areas of external memory, takes only a few recordings, which include the required entry. In the latter case, saying that the file is indexed posledovatelnuyu organization, as well as search includes two parts: direct access to an index of the region drive, and then view the records in this region. Keeping index table assumes the file system. It is clear that the entries in the index files can be of any length. All of this is more applicable to ordinary files, which can be as structured or unstructured. As for other types of files, they have some structure, the well-known file system. For example, the file system should understand the structure of data stored in a file directory or file type symbolic link
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