Computer World. Матросова Т.А. - 58 стр.

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57
The ability to store enormous volumes of data is important, but it would be of
little use if we could not access large amounts of that data quickly. Thus, very large
databases also require speed enhancers. One important speedup is through index
structures. Another way to process more data in a given time is to use parallelism. This
parallelism manifests itself in various ways.
TEXT 4
Read the text and find information about storage manager and buffer manager.
STORAGE AND BUFFER MANAGEMENT
The data of a database normally resides in secondary storage, in today’s com-
puter systems ‘‘secondary storage” generally means magnetic disk. However, to
perform any useful operation on data, that data must be in main memory. It is the
job of the storage manager to control the placement of data on disk and its
movement between disk and main memory.
In a simple database svstem, the storage manager might be nothing more than
the file system of the underlying operating system However, for efficiency purposes,
DBMS’s normally control storage on the disk directly, at least under some
circumstances. The storage manager keeps track of the location of files on the disk
and obtains the block or blocks containing a file on request from the buffer manager.
Recall that disks are generally divided into disk blocks, which are regions of
contiguous storage containing a large number of bytes, perhaps 2
12
or 2
14
(about
4,000 to 16,000 bytes).
The buffer manager is responsible for partitioning the available main memory
into buffers, which are page-sized regions into which disk blocks can be transferred.
Thus, all DBMS components that need information from the disk will interac t w ith
the buffers and the buffer manager, either directly or through the execution engine.
The kinds of information that various components may need include:
1. Data: the contents of the database itself.
2. Metadata: the database schema that describes the structure of, and con
straints on, the database.
3. Statistics: information gathered and stored by the DBMS about data
properties such as the sizes of, and values in, various relations or other
components of the database.
4. Indexes: data structures that support efficient access to the data.
TEXT 5
Read the text. Speak about the advantages of DBMS and RDBMS.
EVOLUTIO N OF DATABAS E SYSTEMS
Since the first database management systems (DBMSs) appeared in the ear ly
1960s, they have evolved in terms of quality (functionality, performance, ease of use,
and so on) and quantity (number of different products). The qualitative evolution has
been driven by two complementary trends: significant progress in the areas of

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