第3章 存储与文件结构

A1 - char(4)
A2 - varchar(20)
A3 - numeric(10,3) A4 - integer A5 - varchar(15)
Database System Concepts
11.18
©Silberschatz, Korth and Sudarshan
Internal Data-Page Organization
Access Time Time = Seek Time + Rotational Delay + Transfer Time
Rule of Thumb
Random I/O: Expensive 20 ms Sequential I/O: Much less 1-5 ms
Database System Concepts
11.8
©Silberschatz, Korth and Sudarshan
存储空间管理
Goals
Enable efficient storage and maintenance of records
Enable fast and efficient access to data Good storage utilization and efficient space reclamation
Alternatives:
Fixed-length records with fixed-length fields Variable-length records with self-describing fields Variable-length records with fixed record maps Variable-length records with fixed and variable
Machine-independent representation
Store attribute values so that they can be compared on both Little machines and Big machines
Database System Concepts
Database System Concepts
11.13
©Silberschatz, Korth and Sudarshan
Physical representຫໍສະໝຸດ tion of attributes
Choice of pre-defined types
Few pre-defined types (e.g., varchar and numeric)
物理存储分层(复习)
返回
Memory System
CPU Die CPU
Registers L1 Cache
L2 Cache Main Memory
Harddisk
Database System Concepts 11.6 ©Silberschatz, Korth and Sudarshan
Memory Hierarchy
Physical representation
Physical representation of attributes
Physical representation of tuples Internal data-page organization Maintenance Mapping records to pages
Questions:
Should data store accommodate dynamic updates? Is storage reuse an important factor? Should access to any record's field be equally fast? Are long tuples required?
portions
Database System Concepts
11.17
©Silberschatz, Korth and Sudarshan
Physical Representation of Tuples
Example used throughout:
r(A1, A2, A3, A4, A5), where
Storage allocation and deallocation
Free-space management
Database System Concepts
11.12
©Silberschatz, Korth and Sudarshan
Problems
Access to records
Block size Size Hit time Backing Store Block replacement Miss penalty L1 Cache 16--32 bytes 16--64 KB 1 Clock Cycle L2 Cache Random 4-20 clock cycles L2 Cache 32--64 bytes 256KB— 8MB 1—4 Clock Cycles Main memory Random 40-200 clock cycles Main memory 4--16 KB 10—40 Clock Cycles Disk Replacement strategies ~6M clock cycles
from a domain
Missing-value approach (field length of 0 indicating null)
Cannot be used in all tuple representations, since not
all of them maintain the "field length"
Database System Concepts
11.19
©Silberschatz, Korth and Sudarshan
Record identifiers
Purpose
Distinguish records from one another
Locate a record via its id as quickly as possible Allow recovery to correlate log entries with records Allow replication services to determine whether two records are, in fact, replicas of the same object
Highly portable, but requires complex
conversions
But can create excessive storage overhead
Many pre-defined types (e.g., all those supported by a programming language and more)
11.15
©Silberschatz, Korth and Sudarshan
Physical representation of attributes
Representing NULL values
Special-value approach
Highly complex and leads to a "loss" of certain values
多,快,好,省
Database System Concepts
11.9
©Silberschatz, Korth and Sudarshan
Improving Access Time of Secondary Storage
Organization of data on disk Disk scheduling algorithms
本章关键问题：
如何正确地实现逻辑地址空间到物理地址空间的映射 如何消除存储设备性能瓶颈 如何管理存储空间
主要内容
物理存储介质概述 物理存储分层
存储器访问
存储空间管理 缓冲区管理器 文件组织(自学) 文件中的记录组织(自学) 数据字典存储(略) 对象到文件的映射(略) 持久性指针的管理(略)
Simple, efficient, few conversions
But not portable
Database System Concepts
11.14
©Silberschatz, Korth and Sudarshan
Physical representation of attributes
data loss on power failure or system crash physical failure of the storage device
Can differentiate storage into:
volatile storage: loses contents when power is switched off 易逝，挥发
Record identification
Direct access based on stable record ids Fast access from one record to the next or prior Other concerns Data encryption(加密) Data compression(压缩) Character codeset(编码集)
non-volatile storage:Contents persist even when power is switched off.Includes secondary and tertiary storage, as well as batterbacked up main-memory. 磁盘，光盘，磁 带，闪存