数据库系统原理(英文)-11-11(精品·公开课件).ppt

April 2010 Database System Concepts - Chapter 11 Storage and File Structures - PART 4 DATA STORAGE AND QUERY Introduction to Chapter11 and Chapter12 In view of physical DB, two issues should be addressed data organization in DBS, i.e. physical storage structure of data — Chapter11 data access in DBS — Chapter12 Chapter 11   Storage and File Structure Main Contents in This Chapter 11-I. Physical storage media in DBS (§11.1—§11.4) storage-device hierarchy, disk, RAID 11-II. Storage access 11-III. File organization at physical level 11-IV. Organization of records in files at logical level, i.e. file structures 11-IV. Data-dictionary (§11.8) §11.1 Physical Storage Media 11-I-1. Storage-device hierarchy (§11.1) As shown in Fig.11.1 Hierarchy primary storage, secondary storage, tertiary storage Performance index reliability, speed, capacity, cost 11-I-2 Magnetic Disk The disk structure is shown in Fig.11.2 Access time is taken as the performance measures of disks the time it takes from when a read or write request is issued to when data transfer begins Access time includes seek time – time it takes to reposition the arm over the correct track. 4 to 10 milliseconds on typical disks rotational latency – time it takes for the sector to be accessed to appear under the head. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) 11-I-2 Magnetic Disk (cont.) Data-transfer rate – the rate at which data can be retrieved from or stored to the disk, e.g. ATA-5: 66 MB/second, SCSI-3: 40 MB/s fiber Channel: 256 MB/s Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure. typically 3 to 5 years Data is transferred between disk and main memory in blocks sizes range from 512 bytes to several kilobytes Fig.11.0.1 11-I-2 Magnetic Disk (cont.) 11-II Storage Access A database file is partitioned into fixed-length storage units called blocks the blocks are units of both storage allocation