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Organization of the file system

External fragmentation of the method is not available. A free block can be used to meet the request. Note that there is no need to declare the file size at the time of creation. This file can grow indefinitely. Coherent allocation is, however, a few significant shortcomings. First, with direct access to the file to find the first block i- need to implement several hits to the disk, reading consistently blocks from 1 to i-1, that is logically related sample records that are physically adjacent sectors that may require a lot of time. A direct consequence of this is the low reliability. The existence of the defective unit in the list of results in the loss of information in the residual part of the file and, potentially, to the loss of disk space allocated by the file. Finally, the pointer on the next block inside need a dedicated unit. The tank unit, which is traditionally degree deuces (many of the programs read and write blocks on the degree of deuces), thus ceases to be a degree of deuces, because index selects a few bytes. Therefore, the method of communication generally list is not used in pure form. Binding list using index Disadvantages previous method can be eliminated by retiring pointer from each disk block and put in a table in memory of the index, which is called the FAT (file allocation table). This scheme had many OS (MS-DOS, OS / 2, MS Windows, etc.) Still, significant that the record in this directory contains only a reference to the first block, and thus you can locate a file regardless of its size. Less of the scheme may be the need for support in the memory of this rather large table. Index nodes The fourth and final method of ascertaining ownership block to the file-link to each file a small table, called the index node (i-node), which lists the attributes and address blocks disc file (see Figure 12.4). Each file has its own block of the index, which is composed of blocks of data. Writing in directories related to the file, the index contains the address block. As fill file pointers to the disk blocks in the index node take meaningful significance. Indexing supports direct access to the file, without prejudice of the external fragmentation.
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