Ebook Fundamentals of database systems (6/E): Part 2
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Ebook Fundamentals of database systems (6/E): Part 2
part7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2, which are typically stored on magnetic disks. This chapter and the next deal with the organization of databases in storage and the techniques for accessing them efficiently using various algorithms, some of which require auxiliary data structures called indexes. These structures are often referred Ebook Fundamentals of database systems (6/E): Part 2 to as physical database file structures, and are at the physical level of the three-schema architecture described in Chapter 2. We start in Section 1Ebook Fundamentals of database systems (6/E): Part 2
7.1 by introducing the concepts of computer storage hierarchies and how they are used in database systems. Section 17.2 is devoted to a description ofpart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2ge technologies, we turn our attention to the methods for physically organizing data on disks. Section 17.3 covers the technique of double buffering, which is used to speed retrieval of multiple disk blocks. In Section17.4we discuss various ways of formatting and storing file records on disk. Sectio Ebook Fundamentals of database systems (6/E): Part 2n17.5discusses the various types of operations that are typically applied to file records. We present three primary methods for organizing file recordEbook Fundamentals of database systems (6/E): Part 2
s on disk: unordered records, in Section 17.6; ordered records, in Section 17.7; and hashed records, in Section 17.8.Section 17.9 briefly introduces fpart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2d databases, which we discussed in Chapter 11. Section 17.10 describes RAID (Redundant Arrays of Inexpensive (or Independent) Disks)—a data storage system architecture that is commonly used in large organizations for better reliability and performance. Finally, in Section 17.11 we describe three dev Ebook Fundamentals of database systems (6/E): Part 2elopments in the storage systems area: storage area networks (SAN), network-583584 Chapter 17 Disk Storage, Basic File Structures, and HashingattachedEbook Fundamentals of database systems (6/E): Part 2
storage (NAS), and iSCSI (Internet SCSI—Small Computer System Interface), the latest technology, which makes storage area networks more affordable wipart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2ter 18 we discuss techniques for creating auxiliary data structures, called indexes, which speed up the search for and retrieval of records. These techniques involve storage of auxiliary data, called index files, in addition to the file records themselves.Chapters 17 and 18 may be browsed through or Ebook Fundamentals of database systems (6/E): Part 2 even omitted by readers who have already studied file organizations and indexing in a separate course. The material covered here, in particular SectiEbook Fundamentals of database systems (6/E): Part 2
ons 17.1 through 17.8, is necessary for understanding Chapters 19 and 20, which deal with query processing and optimization, and database tuning for ipart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2r storage medium. The DBMS software can then retrieve, update, and process this data as needed. Computer storage media form a storage hierarchy that includes two main categories:■Primary storage. This category includes storage media that can be operated on directly by the computer’s central processi Ebook Fundamentals of database systems (6/E): Part 2ng unit (CPU), such as the computer s main memory and smaller but faster cache memories. Primary storage usually provides fast access to data but is oEbook Fundamentals of database systems (6/E): Part 2
f limited storage capacity. Although main memory capacities have been growing rapidly in recent years, they are still more expensive and have less stopart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2Ms, DVDs, and other similar storage media), and tapes. Hard-disk drives are classified as secondary storage, whereas removable media such as optical disks and tapes are considered tertiary storage. These devices usually have a larger capacity, cost less, and provide slower access to data than do pri Ebook Fundamentals of database systems (6/E): Part 2mary storage devices. Data in secondary or tertiary storage cannot be processed directly by the CPU; first it must be copied into primary storage andEbook Fundamentals of database systems (6/E): Part 2
then processed by the CPU.We first give an overview of the various storage devices used for primary and secondary storage in Section 17.1.1 and then dpart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2 system, data resides and is transported throughout a hierarchy of storage media. The highest-speed memory is the most expensive and is therefore available with the least capacity. The lowest-speed memory is offline tape storage, which is essentially available in indefinite storage capacity.17.1 Int Ebook Fundamentals of database systems (6/E): Part 2roduction 585At the primary storage level, the memory hierarchy includes at the most expensive end, cache memory, which is a static RAM (Random AccessEbook Fundamentals of database systems (6/E): Part 2
Memory). Cache memory is typically used by the CPU to speed up execution of program instructions using techniques such as prefetching and pipelining.part7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2is popularly called main memory. The advantage of DRAM is its low cost, which continues to decrease; the drawback is its volatility1 and lower speed compared with static RAM. At the secondary and tertiary storage level, the hierarchy includes magnetic disks, as well as mass storage in the form of CD Ebook Fundamentals of database systems (6/E): Part 2-ROM (Compact Disk-Read-Only Memory) and DVD (Digital Video Disk or Digital Versatile Disk) devices, and finally tapes at the least expensive end of tEbook Fundamentals of database systems (6/E): Part 2
he hierarchy. The storage capacity is measured in kilobytes (Kbyte or 1000 bytes), megabytes (MB or 1 million bytes), gigabytes (GB or 1 billion bytespart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2 of data in physics, astronomy, earth sciences, and other scientific applications.Programs reside and execute in DRAM. Generally, large permanent databases reside on secondary storage, (magnetic disks), and portions of the database are read into and written from buffers in main memory as needed. Now Ebook Fundamentals of database systems (6/E): Part 2adays, personal computers and workstations have large main memories of hundreds of megabytes of RAM and DRAM, so it is becoming possible to load a larEbook Fundamentals of database systems (6/E): Part 2
ge part of the database into main memory. Eight to 16 GB of main memory on a single server is becoming commonplace. In some cases, entire databases capart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2ons that require extremely fast response times. An example is telephone switching applications, which store databases that contain routing and line information in main memory.Between DRAM and magnetic disk storage, another form of memory, flash memory, is becoming common, particularly because it is Ebook Fundamentals of database systems (6/E): Part 2nonvolatile. Flash memories are high-density, high-performance memories using EEPROM (Electrically Erasable Programmable Read-Only Memory) technology.Ebook Fundamentals of database systems (6/E): Part 2
The advantage of flash memory is the fast access speed; the disadvantage is that an entire block must be erased and written over simultaneously. Flaspart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2ring in cameras, MP3 players, cell phones, PDAs, and so on. USB (Universal Serial Bus) flash drives have become the most portable medium for carrying data between personal computers; they have a flash memory storage device integrated with a USB interface.CD-ROM (Compact Disk - Read Only Memory) disk Ebook Fundamentals of database systems (6/E): Part 2s store data optically and are read by a laser. CD-ROMs contain prerecorded data that cannot be overwritten. WORM (Write-Once-Read-Many) disks are a fEbook Fundamentals of database systems (6/E): Part 2
orm of optical storage used for'Volatile memory typically loses its contents in case of a power outage, whereas nonvolatile memory does not586 Chapterpart7File Structures, Indexing, and HashingchapterDisk Storage, Basic File Structures, and HashingDatabases are stored physically as flies of records, Ebook Fundamentals of database systems (6/E): Part 2ility of erasing. They hold about half a gigabyte of data per disk and last much longer than magnetic disks.2 Optical jukebox memories use an array of CD-ROM platters, which are loaded onto drives on demand. Although optical jukeboxes have capacities in the hundreds of gigabytes, their retrieval tim Ebook Fundamentals of database systems (6/E): Part 2es are in the hundreds of milliseconds, quite a bit slower than magnetic disks. This type of storage is continuing to decline because of the rapid decEbook Fundamentals of database systems (6/E): Part 2
rease in cost and increase in capacities of magnetic disks. The DVD is another standard for optical disks allowing4.5to 15 GB of storage per disk. MosGọi ngay
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