第五章 文件管理

OS的四大子系统

Process	Memory	I/O Devices	File
Hardware	Hardware	Hardware	Software

逻辑组织

• 文件管理
• 系统文件的逻辑组织
• 文件目录
• 文件共享
• 文件存储空间的管理

文件管理（File Management）

• File management system is consideredpart of the operating system.文件管理系统被认为是操作系统的一部分。
• Input to applications is by means of a file.应用程序的输入是通过文件进行的。
• Output is saved in a file for long-termstorage. 输出保存在文件中以便长期存储。

Objectives for a File Management System

• Meet the data management needs and
  requirements of the user.
  (符合系统管理数据的需要及用户的需求)
• Guarantee that the data in the file are valid.
  (确保文件中的数据有效)
• Optimize performance.
  (保证系统性能及用户对响应时间的要求)
• Provide I/O support for a variety of storagedevice types.
  (为各种存储设备类型提供I/O支持)
• Minimize or eliminate the potential forlost or destroyed data.
  (减少或避免数据丢失及损坏)
• Provide a standardized set of L/Ointerface routines.
  (提供标准的IO接口)
• Provide I/O support for multiple users.
  (支持多用户环境下多个用户的IO操作)

File Management Functions

• Identify and locate a selected file.
  标识并找到选定的文件。
• Use a directory to describe the location ofall files plus their attributes.
  使用目录来描述所有文件的位置以及它们的属性。
• On a shared system describe user accesscontrol.
  在共享系统上描述用户访问控制。
• Blocking for access to files.
  阻止对文件的访问。
  -Allocate files to free blocks.
  将文件分配给空闲块。
• Manage free storage for available blocks.
  -管理可用区块的空闲空间。

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Device Drivers

• Lowest level.
  最低级别。
• Communicates directly with peripheraldevices.
  直接与外围设备通信。
• Responsible for starting I/O operationson a device.
  负责启动I/O操作设备。
• Processes the completion of an I/orequest.
  处理I/O请求的完成。

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Basic File System

• Physical I/O.
  物理I/O
• Deals with exchanging blocks of data.
  处理数据块的交换。
• Concerned with the placement of blocks.
  与块的放置有关
• Concerned with buffering blocks in main memory.
  与内存中的缓冲块有关。

Basic I/O Supervisor

• Responsible for file I/O initiation andtermination.
• Control structures are maintained.
• Concerned with scheduling access tooptimize performance.
• I/O buffers are assigned and secondarymemory is allocated.

Logical I/O

• Enables users and applications to access records.
  允许用户和应用程序访问记录。

• Provides general-purpose record I/ocapability.
  提供通用记录I/ocapability。

• Maintains basic data about file.
  维护有关文件的基本数据。

Access Method

• Reflect different file structures.
• Different ways to store and process data.
  如Pile、顺序访问、索引顺序访问、索引访问、Hash访问法等访问方法

File Organization

• File organization refers to the logicalstructuring of the records as determinedby the way in which they are accessed.
  文件组织是指由访问方式决定的记录的逻辑结构。
• The physical organization of the file onsecondary storage depends on theblocking strategy and the file allocationstrategy.
  存储上文件的物理组织取决于阻塞策略和文件分配策略。

Criteria for File Organization

• Rapid access
  Needled when accessing a single record.
  Not needed for batch mode.
• Ease of update
  File on CD-ROM will not be updated, so thisis not a concern.
• Economy of storage
  Should be minimum redundancy in the data.- Redundancy can be used to speed accesssuch as an index.
  -Simple maintenance·
• Reliability

File Organization

• Pile file（堆文件)
• Sequential file（顺序文件)
• Indexed sequential file（索引顺序文件)
• Indexed file（索引文件)
• Direct, or Hashed file（直接或hash文件)

堆文件（pile）

• Data are collected in the order theyarrive.
  数据是按到达的顺序收集的。
• Purpose is to accumulate a mass of data and save it.
  目的是积累大量数据并保存它。
• Records may have different fields.
  记录可能有不同的字段。
• No structure.
  无结构文件。

顺序文件（Sequential File）

• Fixed format used for records.
  用于记录的固定格式。
• Records are the same length.
  记录的长度相同。
• All fields the same (order and length).
  所有字段都相同(顺序和长度)。
• Field names and lengths are attributes ofthe file.
  字段名和长度是文件的属性。
  One field is the key filed
  -Uniquely identifies the record.
  -Records are stored in key sequence.
• The only one that is easily stored on tapes as wellas disk .
  唯一 一种可以同时存储在磁带和磁盘上的。
• Typically used in batch applications and theapplications processing of all records.
  通常用于批处理应用程序和处理所有记录的应用程序。
• For querying and/or updating an individlual record，it provides poor performance.
  对于查询和/或更新单个记录，它的性能较差。
• New records are placed in a log file or transactionfile.
  新记录放置在日志文件或transactionfile中。
• Batch update is performed to merge the log filewith the master file.
  执行批更新以将日志文件与主文件合并。

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