PowerPoint PresentationDisk Storage DevicesDisk Storage Devices (cont.)Slide 4Slide 5Slide 6RecordsBlockingFiles of RecordsFiles of Records (cont.)Unordered FilesOrdered FilesOrdered Files (cont.)Average Access TimesHashing TechniquesSlide 16Hashed FilesHashed Files (cont.)Slide 19Slide 20Hashed Files - Overflow HandlingDynamic And Extendible Hashed FilesDynamic And Extendible Hashing (cont.)Extendible HashingSummaryCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-WesleyChapter 17Disk Storage, Basic File Structures, and HashingCopyright © 2011 Ramez Elmasri and Shamkant NavatheDisk Storage DevicesPreferred secondary storage device for high storage capacity and low cost.Data stored as magnetized areas on magnetic disk surfaces.A disk pack contains several magnetic disks connected to a rotating spindle.Disks are divided into concentric circular tracks on each disk surface.Track capacities vary typically from 4 to 50 Kbytes or moreCopyright © 2011 Ramez Elmasri and Shamkant NavatheDisk Storage Devices (cont.)A track is divided into smaller blocks or sectorsbecause it usually contains a large amount of information The division of a track into sectors is hard-coded on the disk surface and cannot be changed.A track is divided into blocks.The block size B is fixed for each system.Typical block sizes range from B=512 bytes to B=4096 bytes.Whole blocks are transferred between disk and main memory for processing.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDisk Storage Devices (cont.)Copyright © 2011 Ramez Elmasri and Shamkant NavatheDisk Storage Devices (cont.)A read-write head moves to the track that contains the block to be transferred.Disk rotation moves the block under the read-write head for reading or writing.A physical disk block (hardware) address consists of:a cylinder number (imaginary collection of tracks of same radius from all recorded surfaces)the track number or surface number (within the cylinder)and block number (within track).Reading or writing a disk block is time consuming because of the seek time s and rotational delay (latency) rd.Double buffering can be used to speed up the transfer of contiguous disk blocks.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDisk Storage Devices (cont.)Copyright © 2011 Ramez Elmasri and Shamkant NavatheRecordsFixed and variable length recordsRecords contain fields which have values of a particular typeE.g., amount, date, time, ageFields themselves may be fixed length or variable lengthVariable length fields can be mixed into one record:Separator characters or length fields are needed so that the record can be “parsed.”Copyright © 2011 Ramez Elmasri and Shamkant NavatheBlockingBlocking: Refers to storing a number of records in one block on the disk.Blocking factor (bfr) refers to the number of records per block. There may be empty space in a block if an integral number of records do not fit in one block.Spanned Records:Refers to records that exceed the size of one or more blocks and hence span a number of blocks.Copyright © 2011 Ramez Elmasri and Shamkant NavatheFiles of RecordsA file is a sequence of records, where each record is a collection of data values (or data items).A file descriptor (or file header) includes information that describes the file, such as the field names and their data types, and the addresses of the file blocks on disk.Records are stored on disk blocks. The blocking factor bfr for a file is the (average) number of file records stored in a disk block.A file can have fixed-length records or variable-length records.Copyright © 2011 Ramez Elmasri and Shamkant NavatheFiles of Records (cont.)File records can be unspanned or spanned Unspanned: no record can span two blocksSpanned: a record can be stored in more than one blockThe physical disk blocks that are allocated to hold the records of a file can be contiguous, linked, or indexed.In a file of fixed-length records, all records have the same format. Usually, unspanned blocking is used with such files.Files of variable-length records require additional information to be stored in each record, such as separator characters and field types.Usually spanned blocking is used with such files.Copyright © 2011 Ramez Elmasri and Shamkant NavatheUnordered FilesAlso called a heap or a pile file.New records are inserted at the end of the file.A linear search through the file records is necessary to search for a record.This requires reading and searching half the file blocks on the average, and is hence quite expensive.Record insertion is quite efficient.Reading the records in order of a particular field requires sorting the file records.Copyright © 2011 Ramez Elmasri and Shamkant NavatheOrdered FilesAlso called a sequential file.File records are kept sorted by the values of an ordering field.Insertion is expensive: records must be inserted in the correct order.It is common to keep a separate unordered overflow (or transaction) file for new records to improve insertion efficiency; this is periodically merged with the main ordered file.A binary search can be used to search for a record on its ordering field value.This requires reading and searching log2 of the file blocks on the average, an improvement over linear search.Reading the records in order of the ordering field is quite efficient.Copyright © 2011 Ramez Elmasri and Shamkant NavatheOrdered Files (cont.)Copyright © 2011 Ramez Elmasri and Shamkant NavatheAverage Access TimesThe following table shows the average access time to access a specific record for a given type of fileCopyright © 2011 Ramez Elmasri and Shamkant NavatheHashing TechniquesInternal HashingFor internal files, hashing is implemented as a hash table through the use of an array of records.External HashingCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Ramez Elmasri and Shamkant NavatheHashed FilesHashing for disk files is called External HashingThe file blocks are divided into M equal-sized buckets, numbered bucket0, bucket1, ..., bucketM-1.Typically, a bucket corresponds to one (or a fixed number of) disk block.One of the file fields is designated to be the hash key of the file.The record with hash key value K is stored in bucket i, where i=h(K), and h is the hashing
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