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.
Next part
|
O
wlCom Software
