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1、Database System Concepts, 6th Ed.Silberschatz, Korth and SudarshanSee www.db- for conditions on re-use Chapter 10: Storage and File StructureSilberschatz, Korth and Sudarshan10.2Database System Concepts - 6th EditionChapter 10: Storage and File StructurenOverview of Physical Storage MedianMagnetic D
2、isksnRAIDnTertiary Storage nStorage AccessnFile OrganizationnOrganization of Records in FilesnData-Dictionary StorageSilberschatz, Korth and Sudarshan10.3Database System Concepts - 6th EditionClassification of Physical Storage MedianSpeed with which data can be accessednCost per unit of datanReliabi
3、lityldata loss on power failure or system crashlphysical failure of the storage devicenCan differentiate storage into:lvolatile storage: loses contents when power is switched offlnon-volatile storage: 4Contents persist even when power is switched off. 4Includes secondary and tertiary storage, as wel
4、l as batter- backed up main-memory.Silberschatz, Korth and Sudarshan10.4Database System Concepts - 6th EditionPhysical Storage MedianCache fastest and most costly form of storage; volatile; managed by the computer system hardware.nMain memory:lfast access (10s to 100s of nanoseconds; 1 nanosecond =
5、109 seconds)lgenerally too small (or too expensive) to store the entire database4capacities of up to a few Gigabytes widely used currently4Capacities have gone up and per-byte costs have decreased steadily and rapidly (roughly factor of 2 every 2 to 3 years)lVolatile contents of main memory are usua
6、lly lost if a power failure or system crash occurs.Silberschatz, Korth and Sudarshan10.5Database System Concepts - 6th EditionPhysical Storage Media (Cont.)nFlash memory lData survives power failurelData can be written at a location only once, but location can be erased and written to again 4Can sup
7、port only a limited number (10K 1M) of write/erase cycles.4Erasing of memory has to be done to an entire bank of memory lReads are roughly as fast as main memorylBut writes are slow (few microseconds), erase is slowerlWidely used in embedded devices such as digital cameras, phones, and USB keysSilbe
8、rschatz, Korth and Sudarshan10.6Database System Concepts - 6th EditionPhysical Storage Media (Cont.)nMagnetic-disklData is stored on spinning disk, and read/written magneticallylPrimary medium for the long-term storage of data; typically stores entire database.lData must be moved from disk to main m
9、emory for access, and written back for storage4Much slower access than main memory (more on this later)ldirect-access possible to read data on disk in any order, unlike magnetic tapelCapacities range up to roughly 1.5 TB as of 20094Much larger capacity and cost/byte than main memory/flash memory4Gro
10、wing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years)lSurvives power failures and system crashes4disk failure can destroy data, but is rareSilberschatz, Korth and Sudarshan10.7Database System Concepts - 6th EditionPhysical Storage Media (Cont.)nOptical storage lno
11、n-volatile, data is read optically from a spinning disk using a laser lCD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular formslBlu-ray disks: 27 GB to 54 GBlWrite-one, read-many (WORM) optical disks used for archival storage (CD-R, DVD-R, DVD+R)lMultiple write versions also available (CD-RW, DVD-R
12、W, DVD+RW, and DVD-RAM)lReads and writes are slower than with magnetic disk lJuke-box systems, with large numbers of removable disks, a few drives, and a mechanism for automatic loading/unloading of disks available for storing large volumes of dataSilberschatz, Korth and Sudarshan10.8Database System
13、 Concepts - 6th EditionPhysical Storage Media (Cont.)nTape storage lnon-volatile, used primarily for backup (to recover from disk failure), and for archival datalsequential-access much slower than disk lvery high capacity (40 to 300 GB tapes available)ltape can be removed from drive storage costs mu
14、ch cheaper than disk, but drives are expensivelTape jukeboxes available for storing massive amounts of data 4hundreds of terabytes (1 terabyte = 109 bytes) to even multiple petabytes (1 petabyte = 1012 bytes)Silberschatz, Korth and Sudarshan10.9Database System Concepts - 6th EditionStorage Hierarchy
15、Silberschatz, Korth and Sudarshan10.10Database System Concepts - 6th EditionStorage Hierarchy (Cont.)nprimary storage: Fastest media but volatile (cache, main memory).nsecondary storage: next level in hierarchy, non-volatile, moderately fast access timelalso called on-line storage lE.g. flash memory
16、, magnetic disksntertiary storage: lowest level in hierarchy, non-volatile, slow access timelalso called off-line storage lE.g. magnetic tape, optical storageSilberschatz, Korth and Sudarshan10.11Database System Concepts - 6th EditionMagnetic Hard Disk MechanismNOTE: Diagram is schematic, and simplifies the structure of actual disk drivesSilberschatz, Korth and Sudarshan10.12Database System Concepts - 6th EditionMagnetic DisksnRead-write head lPositioned very close to the platter surface (almost
