Organization of the file system
All computer applications require storing and updating information.
Possibilities of RAM for data storage is limited. First, the memory is usually
loses its contents after power, and secondly, the volume of data often exceeds
its capacity. Moreover, it is desirable to have information in a non processes.
Therefore adopted store data on external media (usually drives), in units called
files. Most computer systems provided to external memory (secondary), high
capacity, which can store huge amounts of data. However, the characteristics of
access to such devices differ significantly from access to the basic
characteristics of memory. To improve efficiency in the use of these devices
have been developed for specific data structures and algorithms. Files from a
user's perspective Introduction Background data management systems for external
memory begins with the tapes yet, but today they have become with the advent of
magnetic disks. Until then, each application itself to tackle the problem of
naming and structuring data in the external memory. It is difficult to maintain
records of several external storage media permanently stored information. The
Historical step was the move to centralized management systems files. The file
management system assumes the allocation of external memory, display file names
in the address of the external memory and access to data. File system is part of
the operating system, the appointment of which is to organize effective work
with the data stored in external memory and the user interface is convenient
when working with these data. Organize storage of information on a magnetic disk
is not easy. This requires a good knowledge of the device controller disc
features his work registers i.t. e. (usually this is the system I / O component
of the OS, called the driver disk). To save the user from the complexities of
computer equipment and interaction with fashion was clear abstract model of a
file system. Operation record or reading the file conceptually simpler than low-level
operation with the device.
The basic idea of using external memory is as follows. OS divides it into fixed-size
blocks, such as 4096 bytes. From the point of view of each user's file consists
of a set of individual elements, called records (for example, the
characteristics of a facility). Each file stored as a sequence of blocks (not
necessarily adjacent); every integer unit keeps records. In some OS (MS-DOS)
address blocks containing data file can be organized into a coherent list and
made into a separate table in the memory. In other operating system (Unix),
address file data blocks are stored in a separate external memory unit (so-called
index or index node). This admission is called indexation and is the most common
for applications that require random access to records files. The index file is
a list of items, each of which contains a number of blocks in the file and an
indication of the location of the block. In modern OS files unstructured usually
a sequence of bytes (length record equals 1) and reading the next byte
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