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Database System Concepts 11.1 Now Something Different 1st part of the course: Application Oriented 2nd part of the course: Systems Oriented What is “Systems”? A: Not Programming Not programming big things.. Systems = Efficient and safe use of limited resources (e.g., disks) Efficient: resources should be shared, utilized as much as possible Safe: Database System Concepts sharing should not corrupt work of individual jobs 11.2 General Overview Relational model - SQL Formal & commercial query languages Functional Dependencies Normalization Physical Design Indexing Query evaluation Application Oriented Query optimization …. Database System Concepts Systems Oriented 11.3 The systems side of Databases What will we talk about? 1. Data Organization: physical storage strategies to support efficient updates, retrieval 2. Data retrieval: auxiliary data structures to enable efficient retrieval. Techniques for processing queries to ensure efficient retrieval 3. Data Integrity: techniques for implementing Xactions, to ensure safe concurrent access to data. Ensuring data is safe in the presence of system crashes. Database System Concepts 11.4 Data Organization Key points 1. Storage Media “Memory hierarchy” Efficient/reliable transfer of data between disks and main memory Hardware techniques (RAID disks) Software techniques (Buffer mgmt) 2. Storage strategies for relations-file organization Representation of tuples on disks Storage of tuples in pages, clustering. Database System Concepts 11.5 CPU ... M Typical Computer C ... Secondary Storage Database System Concepts 11.6 Storage Media: Players Cache – fastest and most costly form of storage; volatile; managed by the computer system hardware. Main memory: fast access (10s to 100s of nanoseconds; 1 nanosecond = 10–9 seconds) generally too small (or too expensive) to store the entire database Volatile — contents of main memory are usually lost if a power failure or system crash occurs. But… CPU operates only on data in main memory Database System Concepts 11.7 Storage Media: Players Disk Primary medium for the long-term storage of data; typically stores entire database. random-access – possible to read data on disk in any order, unlike magnetic tape Non-volatile: data survive a power failure or a system crash, disk failure less likely than them New technology: Solid State Disks and Flash disks Database System Concepts 11.8 Storage Media: Players Optical storage non-volatile, data is read optically from a spinning disk using a laser CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms Write-one, read-many (WORM) optical disks used for archival storage (CD-R and DVD-R) Multiple write versions also available (CD-RW, DVD-RW, and DVDRAM) Reads and writes are slower than with magnetic disk Tapes Sequential access (very slow) Cheap, high capacity Database System Concepts 11.9 Memory Hierarchy cache Main memory V NV disk Optical storage Tapes Traveling the hierarchy: 1. speed ( higher=faster) 2. cost (lower=cheaper) 3. volatility (between MM and Disk) 4. Data transfer (Main memory the “hub”) 5. Storage classes (P=primary, S=secondary, T=tertiary) Database System Concepts 11.10 Main memory Disk Data Xfers Concerns: 1. Efficiency (speed) can be improved by... a. improving raw data transfer speed b. avoiding untimely data transfer c. avoiding unnecessary data transfer 2. Safety (reliability, availability) can be improved by... a. storing data redundantly Database System Concepts 11.11 Hard Disk Mechanism Database System Concepts 11.12 Read-write head Positioned very close to the platter surface (almost touching it) Surface of platter divided into circular tracks Each track is divided into sectors. A sector is the smallest unit of data that can be read or written. To read/write a sector disk arm swings to position head on right track platter spins continually; data is read/written as sector passes under head Block: a sequence of sectors Cylinder i consists of ith track of all the platters Database System Concepts 11.13 “Typical” Values Diameter: 1 inch 15 inches Cylinders: 100 2000 Surfaces: 1 or 2 (Tracks/cyl) 2 (floppies) 30 Sector Size: 512B 50K Capacity: 360 KB (old floppy) 1.5 TB Database System Concepts 11.14 Performance Measures of Disks Measuring Disk Speed Access time – consists of: Seek time – time it takes to reposition the arm over the correct track. (Rotational) latency time – time it takes for the sector to be accessed to appear under the head. Data-transfer rate – the rate at which data can be retrieved from or stored to the disk. Analogy to taking a bus: 1. Seek time: time to get to bus stop 2. Latency time; time spent waiting at bus stop 3. Data transfer time: time spent riding the bus Database System Concepts 11.15