中英对照文件系统.htm

软考英语精典短文每日一练汇总 软考英语精典短文每日一练汇总

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      <DIV id=cont_right style="WIDTH: 495px">A file is an abstract data type 
      defined and implemented by the operating system．A file is a sequence of 
      logical records．A logical record may be a byte，a line（fixed or variable 
      length），or a more complex data item．The operating system may specifically 
      support various record types or may leave that to the application 
      program．<BR><BR>The major problem for the operating system is to map the 
      logical file concept onto physical storage devices such as magnetic tape 
      or disk．Since the physical record size of the devices may not be the same 
      as the logical record size，it may be necessary to block logical records 
      into physical records．Again，this task may be supported by the operating 
      system or left for the application program．<BR><BR>Tape-based file systems 
      are quite constrained；most file systems are disk based．Tapes are commonly 
      used for data transport between machines，or for backup or archival 
      storage．<BR><BR>On a disk system，files may be either sequential access or 
      direct access．Files can be allocated space on the disk in three 
      ways：contiguous，linked，or indexed．Contiguous allocation can suffer from 
      external fragmentation．Direct access files cannot be supported with linked 
      allocation．Indexed allocation may require substantial overhead for its 
      index block．<BR><BR>Each device in a file system keeps a volume table of 
      contents or device directory listing the location of the files on the 
      device．In addition，it is useful to create directories to allow files to be 
      organized．A single-level directory causes naming problems，since each file 
      must have unique name．A two-level directory solves the problem by creating 
      a separate directory for each user．Users have their own directory 
      containing their own files．<BR><BR>The natural generalization of a 
      two-level directory is a tree-structured directory．A tree-structured 
      directory allows users to create subdirectories to organize their 
      files．Acyclic graph directory structures allow subdirectories and files to 
      be shared，but complicate searching and deletion．A general graph structure 
      allows complete flexibility in the sharing of files and directories，but 
      sometimes requires garbage[1] collection to recover unused disk 
      space．<BR><BR>Since files are the main information storage mechanism in 
      most computer systems，file protection is needed．Access to files can be 
      controlled separately for each type of 
      access：read，write，execute，append，list directory，and so on[2]．File 
      protection can be provided by passwords，access lists，or special ad hoc[3] 
      techniques．<BR><BR>File systems are often implemented in a layered or 
      modular structure．The lower levels deal with the physical properties of 
      storage devices．Upper levels deal with symbolic file names and logical 
      properties of file．Intermediate levels map the logical file concepts into 
      physical device 
      properties．<BR><BR>文件是操作系统定义和实现的抽象数据类型。文件是逻辑记录的序列。逻辑记录可以是一个字节、一行字符（固定长或变长），或者一个更复杂的数据项。操作系统可以具体地支持各种记录类型或者交给应用程序支持。<BR><BR>操作系统的主要任务是将逻辑文件概念映射到诸如磁带或磁盘一类的物理存储器设备上。由于存储设备的物理记录大小与逻辑记录大小可能不一样，所以，有可能需要将逻辑记录按物理记录大小分块。同样，这个任务又可由操作系统或者交给应用程序支持。<BR><BR>基于磁带的文件系统相当受限，大多数文件系统是基于磁盘的。磁带通常用于机器间的数据传送、用做备份或档案存储。<BR><BR>在磁盘系统中，文件可以顺序存取或直接存取。文件在磁盘上分配空间有3种方式：邻接的、链接的或索引的方式。邻接分配能承受外部碎片；直接访问文件不能为链接分配所支持；索引式分配又可能因其索引块而需要大量额外开销。<BR><BR>文件系统中的每个设备都保留一个卷目录表或设备目录，以列出设备上文件的位置。另外，建立目录对文件的组织非常有用，因为每个文件都必须具有惟一的名称，所以单级目录会引起命名问题。二级目录通过为每个用户建立一个单独的目录解决了这个问题。用户各有包含自己文件的目录。<BR><BR>两级目录的自然推广是树结构式目录。树结构式目录允许用户建立子目录以组织他们的文件。非循环图目录结构允许共享子目录和文件，但这使得搜索和删除复杂化。一般的图形结构具有文件和目录共享的全部灵活性，但有时需要回收“垃圾”单元以恢复未使用的磁盘空间。<BR><BR>由于文件是大多数计算机系统主要的信息存储机构，所以需要保护文件，对文件的每一种类型的访问：读、写、执行、附加、列目录等可分别加以控制。文件保护可由口令、存取表或专用的特定技术实现。<BR><BR>文件系统常常以层次或模块结构来实现。低层结构处理存储设备的物理特性，高层结构处理符号文件名和文件的逻辑特性，而中间结构将逻辑文件概念映射成物理设备特性。<BR><BR></DIV></TD></TR></TBODY></TABLE>
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