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Unformatted text preview: ed by using the information in index block.
The method, however, has the following drawbacks:
1. It suffers from wastage of disk space especially when most files are small. This
is because each file is allocated one index block irrespective of the number of data
blocks allocated to the file. Hence if a file has one or two data blocks, an entire
index block is used for storing just one or two pointers, leaving the index block
area largely unused.
One way to take care of this problem is to have enough space in the directory so
that the first few (say n) block addresses can be stored in the directory itself. So
for small files, all the necessary information is right in the directory. An index
block is allocated for a file only when the file requires more than n blocks for
storing its data.
2. It suffers from the problem of handling very large files. This is because if the
index block has space that is sufficient for storing only m disk block addresses,
then how can we store files whose size is larger than m disk blocks? This problem
is often taken care of by using the method of multilevel index. In this method, one
of the addresses in the index block is the address of a disk block called the secondlevel index block. This block contains pointers to additional disk blocks allocated
to the file. This approach could be continued to a third or fourth level, but two
levels of indexes are generally sufficient.
For processing data, a computer must first input data and programs for which it
needs input devices. Similarly, to produce the results of processing, it needs output
devices. Chapter 9 provided a description of several of these I/O devices. The
device management module of an operating system takes care of controlling all the
I/O devices of a computer system and provides a simple and easy to use interface
to these devices. How these requirements are taken care of by an operating system
is described below.
Controlling I/O Devices
We saw in Chapter 9 that device controllers are used for connecting I/O devices to
a computer system. Each device co...
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- Spring '14