Specifications of a typical disk


Descriptions


Model number


ST136403LC

ST318203LC


Model name

Cheetah 36

Cheetah 18LP


Form Factor (width)

3.5-inch

3.5-inch


Weight

1.04 Kg

0.59 Kg




















Capacity/Interface


Formatted capacity

36.4 Gbytes, formatted

18.2 Gbytes, formatted


18.2 Gbytes, formatted

80-pin Ultra-2 SCSI

80-pin Ultra-2 SCSI


Configuration


Number of Discs (physical)

12

6


Number of heads (physical)

24

12


Total cylinders (SCSI only)

9,772

9,801


Total tracks (SCSI only)

N/A

117,612


Bytes per sector

512

512


Track Density (TPI)

N/A tracks/inch

12,580 tracks/inch


Recording Density (BPI, max)

N/A bits/inch

258,048 bits/inch




Performance

Transfer Rates


Internal Transfer Rate (min)

193 Mbits/sec

193 Mbits/sec


Internal Transfer Rate (max)

308 Mbits/sec

308 Mbits/sec


Formatted Int transfer rate (min)

18 Mbits/sec

18 Mbits/sec


Formatted Int transfer rate (max)

28 Mbits/sec

28 Mbits/sec


External (I/O) Transfer Rate (max)

80 Mbits/sec

80 Mbits/sec





Seek Times



Average seek time, read

5.7 msec typical

5.2 msec typical


Average seek time, write

6.5 msec typical

6 msec typical


Track-to-track seek, read

0.6 msec typical

0.6 msec typical


Track-to-track seek, write

0.9 msec typical

0.9 msec typical


Full disc seek, read

12 msec typical

12 msec typical


Full disc seek, write

13 msec typical

13 msec typical


Average Latency

2.99 msec

2.99 msec





Other


Default buffer (cache) size

1,024 Kbytes

1,024 Kbytes


Spindle Speed

10,000 RPM

10,016 RPM


Nonrecoverable error rate

1 per bits read

1 per bits read


Seek errors (SCSI)

1 per bits read

1 per bits read

Secondary storage devices

Today and Yesterday I will describe some characteristics of magnetic disk and magnetic tape storage devices. Readers who have studied these devices already may just browse through this post.

From this post I will describe two topic....

1) Hardware Description of Disk Devices

2) Magnetic Tape storage devices

Hardware Description of Disk Devices

Magnetic disks are used for storing large amounts of data. The most basic unit of data on the disk is a single bit of information. By magnetizing an area on disk in certain ways, one can make it represent abit value of either 0 (zero) or 1 (one). To code information, bits are grouped into bytes (or characters). Byte sizes are typically 4 to 8 bits, depending on the computer and the device. We assume that one character is stored in a single byte, and we use the terms byte and character interchangeably. The capacity of a disk is the number of bytes it can store, which is usually very large. Small floppy disks used with microcomputers typically hold from 400 Kbytes to 1.5 Mbytes; hard disks for micros typically hold from several hundred Mbytes up to a few Gbytes; and large disk packs used with minicomputers and mainframes have capacities that range up to a few tens or hundreds of Gbytes. Disk capacities continue to grow as technology improves. A disk is single-sided if it stores information on only one of its surfaces and double-sided if both surfaces are used. To increase storage capacity, disks are assembled into a disk pack , which may include many disks and hence many surfaces. Information is stored on a disk surface in concentric circles of small width, each having a distinct diameter. Each circle is called a track. For disk packs, the tracks with the same diameter on the various surfaces are called a cylinder because of the shape they would form if connected in space. The concept of a cylinder is important because data stored on one cylinder can be retrieved much faster than if it were distributed among different cylinders.

The number of tracks on a disk ranges from a few hundred to a few thousand, and the capacity of each track typically ranges from tens of Kbytes to 150 Kbytes. Because a track usually contains a large amount of information, it is divided into smaller blocks or sectors. The division of a track into sectors is hard-coded on the disk surface and cannot be changed. One type of sector organization calls a portion of a track that subtends a fixed angle at the center as a sector . Several other sector organizations are possible, one of which is to have the sectors subtend smaller angles at the center as one moves away, thus maintaining a uniform density of recording . Not all disks have their tracks divided into sectors.

The division of a track into equal-sized disk blocks (or pages) is set by the operating system during disk formatting (or initialization). Block size is fixed during initialization and cannot be changed dynamically. Typical disk block sizes range from 512 to 4096 bytes. A disk with hard-coded sectors often has the sectors subdivided into blocks during initialization. Blocks are separated by fixed-size interblock gaps, which include specially coded control information written during disk initialization. This information is used to determine which block on the track follows each interblock gap.

In my next post I will represent specifications of a typical disk.

Once the design of a database is completed and a DBMS is chosen to implement the database, the first order of the day is to specify conceptual and internal schemas for the database and any mappings between the two. In many DBMSs where no strict separation of levels is maintained, one language, called the data definition language (DDL), is used by the DBA and by database designers to define both schemas. The DBMS will have a DDL compiler whose function is to process DDL statements in order to identify descriptions of the schema constructs and to store the schema description in the DBMS catalog.

 

 

In DBMSs where a clear separation is maintained between the conceptual and internal levels, the DDL is used to specify the conceptual schema only. Another language, the storage definition language (SDL), is used to specify the internal schema. The mappings between the two schemas may be specified in either one of these languages. For a true three-schema architecture, we would need a third language, the view definition language (VDL), to specify user views and their mappings to the conceptual schema, but in most DBMSs the DDL is used to define both conceptual and external schemas.

 

Once the database schemas are compiled and the database is populated with data, users must have some means to manipulate the database. Typical manipulations include retrieval, insertion, deletion, and modification of the data. The DBMS provides a data manipulation language (DML) for these purposes.

 

 

In current DBMSs, the preceding types of languages are usually not considered distinct languages; rather, a comprehensive integrated language is used that includes constructs for conceptual schema definition, view definition, and data manipulation. Storage definition is typically kept separate, since it is used for defining physical storage structures to fine-tune the performance of the database system, and it is usually utilized by the DBA staff. A typical example of a comprehensive database language is the SQL relational database language , which represents a combination of DDL, VDL, and DML, as well as statements for constraint specification and schema evolution. The SDL was a component in earlier versions of SQL but has been removed from the language to keep it at the conceptual and external levels only.

There are two main types of DMLs. A high-level or nonprocedural DML can be used on its own to specify complex database operations in a concise manner. Many DBMSs allow high-level DML statements either to be entered interactively from a terminal (or monitor) or to be embedded in a general-purpose programming language. In the latter case, DML statements must be identified within the program so that they can be extracted by a pre-compiler and processed by the DBMS. A low-level or procedural DML must be embedded in a general-purpose programming language. This type of DML typically retrieves individual records or objects from the database and processes each separately. Hence, it needs to use programming language constructs, such as looping, to retrieve and process each record from a set of records. Low-level DMLs are also called record-at-a-time DMLs because of this property. High-level DMLs, such as SQL, can specify and retrieve many records in a single DML statement and are hence called set-at-a-time or set-oriented DMLs. A query in a high-level DML often specifies which data to retrieve rather than how to retrieve it; hence, such languages are also called declarative.

 

Whenever DML commands, whether high-level or low-level, are embedded in a general-purpose programming language, that language is called the host language and the DML is called the data sublanguage . On the other hand, a high-level DML used in a stand-alone interactive manner is called a query language. In general, both retrieval and update commands of a high-level DML may be used interactively and are hence considered part of the query language .

 

Casual end users typically use a high-level query language to specify their requests, whereas programmers use the DML in its embedded form. For naive and parametric users, there usually are user-friendly interfaces for interacting with the database; these can also be used by casual users or others who do not want to learn the details of a high-level query language. We discuss these types of interfaces next.

 

 

 

Hello friends.. It is my first post on Database management system.my blog is dedicated to database management for the people who want to know the basics of database and In this topic I m going to cover the advantages of DBMS in brief.

1. Data independence :-
   Application programs should be as independent as possible from details of data representation and storage. The DBMS can provide an
   

abstract view of the data to insulate application code from such details.

 

2)Efficient Data access:-
A DBMS provides technique to store the data and received data more efficiently .This technique is more useful when we are accessing data from the external device.

 

 

3) Data Integrity and Security:- If data is always accessed through the DBMS, the DBMS can enforce integrity constraints on the data. For example, before inserting

salary information for an employee, the DBMS can check that the department budget is not exceeded. Also, the DBMS can enforce access controls that govern

what data is visible to di_erent classes of users.

 

 

4)Data Administration:- When several users share the data, centralizing the administration of data can o_er signi_cant improvements. Experienced professionals

who understand the nature of the data being managed, and how di_erent groups of users use it, can be responsible for organizing the data representation to minimize

redundancy and for fine-tuning the storage of the data to make retrieval efficient.

 

 

5) Concurrent Access and crash recovery :- A DBMS schedules concurrent accesses to the data in such a manner that users can think of the data as being

accessed by only one user at a time. Further, the DBMS protects users from the effects of system failures.

 

 

6) Reducing Application Development time:-
Clearly, the DBMS supports many important functions that are common to many applications accessing data stored

in the DBMS. This, in conjunction with the high-level interface to the data, facilitates quick development of applications. Such applications are also likely to be

more robust than applications developed from scratch because many important tasks are handled by the DBMS instead of being implemented by the application.

 

 

 

7) Permitting Interencing and Actions Using Rules :- Some database systems provide capabilities for defining deduction rules for inferencing new information from the stored database facts. Such systems are called deductive database systems. For example, there may be complex rules in the miniworld application for determining when a student is on probation. These can be specified declaratively as rules, which when compiled and maintained by the DBMS can determine all students on probation. In a traditional DBMS, an explicit procedural program code would have to be written to support such applications. But if the miniworld rules change, it is generally more convenient to change the declared deduction rules than to recode procedural programs. More powerful functionality is provided by active database systems, which provide active rules that can automatically initiate actions.

 

 

 

8)Providing Multiple Users Interfaces :- Because many types of users with varying levels of technical knowledge use a database, a DBMS should provide a variety of user interfaces. These include query languages for casual users; programming language interfaces for application programmers; forms and command codes for parametric users; and menu-driven interfaces and natural language interfaces for stand-alone users. Both forms-style interfaces and menu-driven interfaces are commonly known as graphical user interfaces (GUIs). Many specialized languages and environments exist for specifying GUIs. Capabilities for providing World Wide Web access to a database—or web-enabling a database—are also becoming increasingly common.

 

 

9)Representing Complex Relationships Among Data :- A database may include numerous varieties of data that are interrelated in many ways. Consider the example shown in Figure 01.02. The record for Brown in the student file is related to four records in the GRADE_REPORT file. Similarly, each section record is related to one course record as well as to a number of GRADE_REPORT records—one for each student who completed that section. A DBMS must have the capability to represent a variety of complex relationships among the data as well as to retrieve and update related data easily and efficiently.

 

 

10)Providing Backup And Recovery:- A DBMS must provide facilities for recovering from hardware or software failures. The backup and recovery subsystem of the DBMS is responsible for recovery. For example, if the computer system fails in the middle of a complex update program, the recovery subsystem is responsible for making sure that the database is restored to the state it was in before the program started executing. Alternatively, the recovery subsystem could ensure that the program is resumed from the point at which it was interrupted so that its full effect is recorded in the database.