英语翻译

The Physical Layer

The physical layer contains the data files that hold all the data for the database.Nearly all modern DBMSs allow the database to be stored in multiple data files,which are usually spread out over multiple physical disk drives.With this arrangement,the disk drives can work in parallel for maximum performance.A notable exception among the DBMSs used as examples in this book is Microsoft Access,which stores the entire database in a single physical file.While simplifying database use on a single-user personal computer system,this arrangement limits the ability of the DBMS to scale to accommodate many concurrent users of the database,making it inappropriate as a solution for large enterprise systems.In all fairness,Microsoft Access was not designed to be a robust enterprise class DBMS.I have included it in discussions in this book not because it competes with products such as Oracle and SQL Server,but because it's a great example of a personal DBMS with a user interface that makes learning database concepts easy and fun.

The database user does not need to understand how the data is actually stored within the data files or even which file contains the data items) of interest.In most organizations,a technician known as a database administrator (DBA) handles the details of installing and configuring the database software and data files and making the database available to users.The DBMS works with the computer's operating system to manage the data files automatically,including all file opening,closing,reading,and writing operations.The database user should not be required to refer to physical data files when using a database,which is in sharp contrast with spreadsheets and word processing,where the user must consciously save the documents) and choose file names and storage locations.Many of the personal computer-based DBMSs are exceptions to this tenet because the user is required to locate and open a physical file as part of the process of signing on to the DBMS.Conversely,with enterprise class DBMSs (such as Oracle,Sybase ASE,Microsoft SQL Server,and MySQL),the physical files are managed automatically and the database user never needs to refer to them when using the database.