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A database management system (DBMS) is a collection of programs that enables you to store, extract information from a database. There are many different types of database management systems, ranging from small systems that run on personal computers to huge systems that run on mainframes.
in the computer there is the seek time and this is the time needed to move the read-write heads from one position to another and we have also the the relational delay and this occurs because the data we want may not be directly under the read-write heads, even though they are located over the correct track.
Computers store data in a file, which can be defined simply as a collection of data. A computer file is organized in a particular way with a well-defined structure for the information in the file.
A computer file consists of a collection of records, each of which is made up of fields. The various fields consist of groups of characters, as described below.
The smallest unit of storage is the byte, which consists of 8 bits. This byte can represent numbers, characters, or parts of an image.
The unit of interest in processing business data is the character, for example, the number 9 or the letter A. (We generally do not work directly with characters but rather with groups of characters that have some intrinsic meaning, such as Smith or 599.
These groupings of characters are called fields, and we identify them with a name. Smith is an employee's surname and 599 is Smith's department number.)
Groups of fields are combined to form a logical record, such as the one shown in Figure 10- 1. This logical record contains all the data of interest about some entity. In this example, it has all the data in the file about an individual employee.
Keys are essential in a relational database. They are statements of UNIQUENESS. Keys are what is used by the RDMS to tell the rows of a table apart from each other, and how rows in one table relate to corresponding rows in another table.
A key to a record is a specific field of interest that will be used as a basis for storing and retrieving data. Many files are organized on a key: The last name is the primary key for a telephone book; that is, the telephone book is arranged in alphabetical order based on telephone subscribers' last names.
A secondary key, in the case of the telephone book, is the person's first name or initial. The telephone book, then, is arranged in sequence on the primary key (last name), and within the primary key it is arranged in order by the secondary key (first name).
Fields designated as keys are also used as a basis for retrieving information from a file. For example, an inventory part number may be the key for retrieving information from an inventory file about the quantity of the part on hand.
There are two major types of files: sequential and direct access.
Sequential files were the first type of secondary storage. All records are kept in some sequence such as numerical order by Social Security numbers. Most of us will encounter sequential access files only in special circumstances. Records in this type of file are located one after another according to a given sequence. For example, the record with payroll number 1 is followed by the record with number 2, etc. With a sequential file, you cannot find a specific record, such as the person with payroll number 1 7, unless you read the entire file until you locate a record with payroll number 127. On the average, if there are n records in the file, you will read nl2 records to find the one you are seeking.
A direct-access file uses a physical medium and programming, which facilitate the storage and retrieval of specific records. These files are at the heart of database management systems and most of today’s file storage technology. The reservations data in an airline's reservations system are stored on direct-access files.
The most common device for storing direct-access files is the magnetic disk drive (see Figure 10-2).
One type of disk consists of a series of platters mounted on a spindle. The top and bottom of each platter (except for the very top and bottom ones) are coated with a magnetic material like that on a music cassette tape.
Read and write heads are fitted between the platters. They float on a cushion of air created by the rotation of the disk and do not actually touch the surface of the platter.
By moving the heads in and out, we can access any spot on the rotating disk. Holding the head in one place traces a track on the disk as the platter rotates under the read-write head.
The maximum block size or physical record size for a disk file is limited by the physical capacity of each track. Looking down from the top of the disk, the tracks on each surface form a cylinder.
When using a disk file sequentially, we write on a given track of the first platter, then on the same track of the second platter, and so on. This strategy minimizes the access time because the heads make the minimum possible movement.
Each track on the disk has an address. Usually, manufacturer-supplied software lets us specify a file and record size, and then retrieve a specific record. The records are numbered 1 through n, where n is the number of records in the file. Thus, we can treat a file as consisting of a group of separately numbered records without concern for the physical track address where the record is stored. The software associates the track address with a logical record and finds the desired record for MS.
The total access time to read or write is made up of two components:
Seek time is the time needed to move the read-write heads from one position to another.
Rotational delay occurs because the data we want may not be directly under the read-write heads, even though they are located over the correct track. We have to wait for the disk to revolve to the beginning of the desired data.
A Solid State Drive (SSD)
A solid-state drive (SSD) (also known as a solid-state disk or electronic disk though it contains no actual "disk" of any kind or motors to "drive" the disks) is a data storage device using integrated circuit assemblies as memory to store data persistently.
SSD technology uses electronic interfaces compatible with traditional block input/output (I/O) hard disk drives, thus permitting simple replacement in common applications
SSDs have no moving mechanical components. This distinguishes them from traditional electromechanical magnetic disks such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads.
Compared with electromechanical disks, SSDs are typically more resistant to physical shock, run silently, have lower access time, and less latency.
However, SSDs are considerably more expensive per unit of storage than HDDs.
Finding Data on the File
In a sequential file, finding the data you want is not too difficult, though it may be time-consuming. Each record is in a sequence, so you simply read the file until you get to the location of the record of interest. (This is the reason why sequential files are associated with batch processing and usually with magnetic tape. You update the file at one point in time and make all the changes, reading the file just once and creating a new version.)
The major advantage of the direct-access file is, as its name implies, that you can locate any record in the file in roughly the same short (milliseconds) period. For example, when you call an airline, they want to access the inventory of seats for the flight you want to take on the date you want to fly without having you or the agent wait on the phone.
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