13.1 through 13.5, 13.10 and 13.11

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474 Chapter 13 Disk Storage, Basic File Structures, and HashingFigure 13.3 illustrates how two processes can proceed in parallei. Processes A ancl iiare running concurrently in an interleaved fashion, whereas processes C and D ar.lunr.ring concurrently in a parallel fashion. When a singie CPU controls rrultil.i.processes, parallel executiolt is not possible. However, the processes can still rur:concurrently in an ir.rterleaved way. Buffering is most useful when processes can nulconcurrently in a parallel fashion, either becar,rse a separate disk I/O processor i.available or because multiple CPU processors exist.Figure 13.4 illustrates how reading and processing can proceed in parallel when thctirne required to process a disk block in mernory is less than the time required t(rread the next block and fill a buffer. The CPU can start processing a biock once ir.transfer to main memory is completed; at the same time, the disk I/O processor canbe reading and transf'erring the next block into a different buffer. This techniqr.re i:called double buffering and can also be used to read a corrtinuous stream of block.Figure 13.3Interleaved concurrencyversus parallel executionlnterleaved concurrencyof operations A and BParallel execution ofoperations C and DiA',AIIllrBlIrBIIItt.Iit^_-__-|>TimeFigure 13.4Use of two buffers, A and B, for readinq from disk,Disk Block:t/o;iFiil At+1FiII Bt+2F|IIAIri+ sFillAt,ri+4Fill ADisk Block:PROCESSING:ll'tt i t i+1i+2lProcessA I ProcessB I ProcessA€lrllIIIIII i+3i Process Bl{.--------------IIIII i+4' Process AlHII______>Time
13.4 Placinq File Records on Disk475\andBr,.l D arenultiple,till runcan run!'SSOr lShen the,rired toonce its,sor canrique isi blocks_>A;SA----->_'.'->mefrom disk to memory. Double buffering perrnits continuous reading or writing ofdata on consecutive disk blocks, which elirninates the seek time and rotational delayfor all but the first block transfer. Moreover, data is kept ready for processing, thusreducing the lvaiting time in the programs.13.4 Placing File Records on DiskIn this section, we define the concepts of records, record types, and files. Then rvediscuss techniques for placing file records on disk.13.4.1 Records and Record TypesData is usuallystored in the form of records. Each record consists of a collection ofrelated data values or items, where each value is formed of one or more bytes ar-rdcorresponds to a particular field of the record. Records usually describe entities andtheir attributes. For example, an EMPLOYEE record represents an employee entity,and each field value in the record specifies some attribute of that employee, such asName, Birlh date, Salary, or Supervisor. A collection of field names and their correspondingdata types constitutes a record type or record format definition. A datatype, associated with each field, specifies the types of values a field can take.The data type of a field is usually one of the standard data types used in programming.These include nuneric (integer, long integer, or floating point), string ofcharacters (fixed-length or varying), Boolean (having 0 and I or TRUE and FALSEvalues only), and sometimes specially coded date and time data types. The numberof bytes required for each data type is fixed for a given computer system. An integermayrequire4bytes,alonginteger8bytes,arealnumber4bytes,aBooleanlbyte,a date l0 bytes (assuming a format of YY\Y-MM-DD), and a fixed-length string ofk characters k bytes. Variable-length strings may require as many bytes as there arecharacters in each field value. For exampie, an EMPLOYEE record type may bedefined-using the C progran-rrning language notation-as the following structure:structIi^hrrchar'in+employee{n^m6r ?n I .ssn[9]ic^l^rrr.int job_code;char department [ 20 ] ;In recent database applications, the need may arise for storing data items that consistof large unstructured objects, which represent images, digitized video or audiostreams, or free text. These are referred to as BLOBS (Binary Large Objects). ABLOB data item is typically stored separately from its record in a pool of disk blocks,and a oointer to the BLOB is included in the record.
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Page 3 and 4: 13.1 Introduction!( ) frl \.i()tl t
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13.1 through 13.5, 13.10 and 13.11

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