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Files and file systems

The first computers each application, which was required to store data in external memory, to determine the location of each portion of data on magnetic tape or drum and served exchanges between operational and external memory with the help of computer commands or call the relevant programmes operating system. Such an operation does not allow or significantly hampered maintenance on one of several foreign media archives long stored information. In addition, each application had to deal with naming of the parts and their structure in external memory. Revolutionary step was a shift to centralized management systems files. In terms of application file - a region known as external memory, which can be saved and from which you can read the data. The rules for naming files, way to access data stored in a file, and the structure of the data depends on the specific management system files and, possibly, of the type of file. File management system assumes the distribution of external memory, display file names to the appropriate address external memory and access to data. In all file systems explicitly or implicitly allocated some basic level, providing work with files representing a set of directly addressed in the address space file blocks. These logical blocks multiple of the amount of physical disk block, and usually chosen equal virtual memory page, which is supported by computer equipment in conjunction with the operating system. In some basic level file systems available to the user, but more often it rose higher, standard for all users. In practice, have proliferated in two basic approaches. In the first approach, users represent file as a sequence of records. Each record - this sequence of bytes permanent or variable size. Entries can be read or write, or to position consistently file on record with a specified number. Some file systems allow structured records in fields and declare some fields keys record. In these file systems can require sample records from its file on a given key. Naturally, in this case supports a file system in the same (or another - Service) core file additional office invisible user data structures. Common ways of organizing key files are based on hashing and Technology B-trees. The second approach is that any file submitted as a sequence of bytes. Because you can read the file specified number of bytes from its beginning or making his pre-positioning of bytes with the specified number. You can also write specified number of bytes at the end of file, or making his pre-positioning. Modern file systems support multilevel naming files from the maintenance of additional external memory files with a special structure - catalogues. Each directory contains the names of directories and / or files contained in this directory. Thus, the full name of the file consists of a list of names of directories, plus the file name in the directory containing this file directly. The difference between the way for naming files in different file systems is how to begin this chain of names. In many file management systems require that each archive files - a tree full of manuals, are entirely located in one package or logical disk drive - the section of physical disk package, submitted by means of operating system as a separate drive. In this case, the full file name starts with the name of the disk device. This method is used in naming the company file systems DEC. Very close to naming files in this way are file systems of personal computers. This organization is called the maintenance filenaming isolated file systems. Another option is the last of the totality of directories and files as a single tree. Full name of the file name starts with the root directory and the user is not obliged to care about the installation disk device on any specific discs. The very system by searching the file by name, asks to install the needed discs. Such a file system called a completely centralized. Centralized file system is largely isolated systems easier, as they file management system assumes more routine work. But in such systems has had a substantial problem, if someone is required to defer podderevo file system to another computer installation. A compromise solution used in file systems, OS UNIX. At the basic level in these file systems are supported isolated archives files. One of these archives declared root disk. After running the system could "mount a" root and a number of separate file systems in a common file system. Since the file systems are a common data repository, generally belonging to different users, files management system should provide authorization to access files. Such an approach is that in relation to each registered user of the computer system for each existing file indicates the action that he allowed or prohibited. In most modern systems of government files every registered user corresponds to a pair of integral identifiers - identifier group to which the user ID and his own group. In this case, every time a file is kept full ID, which created the file, noting what actions with the file may make himself what actions with the file available to other users of the same group, and that could do with the file, users of other groups. This information is compact, when checking is not required large quantities of action and a way to control access sufficient in most cases. If your operating system supports multi-mode, it is real situation where two or more users simultaneously trying to work with the same file. If these people are going to only read the file, then no problem arises. But if one of them will change the file, then to perform the work of this group requires mutual synchronization of the operating system. Consider the possible areas of application file systems. First and foremost, of course, the files used for storing text data: documents, texts of programmes and the like. Such files typically are formed and modified with text editors. The structure is quite simple text files - is a sequence of records containing lines of text or sequence of bytes, among which there are special characters, for example, the end of the line. Files with the texts of programs are used as input texts compilers, which in turn create files containing object modules. In terms of file system, object files also have a simpler structure - a sequence of records or bytes. The logical structure of the same object module unknown file system, and supported programs on the system programming. Similarly, the case files compiled editors ties and containing images of a running program. The logical structure of such files remain known only to editor ties and loader - program operating system. Roughly the same situation with files containing graphic and audio information. Thus, file systems typically provide storage poorly structured information, leaving the structure further applications. In the cases listed above, this is a positive feature, because it allows when developing software, using simple and relatively cheap means of a file system, to organize the structure of data storage, the most relevant specifics of Applied area.
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