第十讲 文件管理

1. 第十讲 文件管理 中国科学技术大学计算机系 陈香兰 xlanchen@ustc.edu.cn Fall2013

2. 内容提要 • 文件定义、属性、操作和类型 • 文件的逻辑结构及其访问模式 • 文件系统的组织 • 目录结构 • 文件保护 • 文件系统的层次结构 • 外存分配算法和空闲空间的管理

3. 内容提要 • 文件定义、属性、操作和类型 • 文件的逻辑结构及其访问模式 • 文件系统的组织 • 目录结构 • 文件保护 • 文件系统的层次结构 • 外存分配算法和空闲空间的管理

4. File-system Files ＋ Directory 逻辑视图 文件结构 目录结构 逻辑结构和组织 文件系统结构 文件的盘块组织 盘块分配 空闲盘块组织 物理结构和组织

5. 文件的定义 • 文件系统在物理存储系统中建立了一个统一的逻辑上的视图 • 文件是最小的逻辑存储单元 • A named collection of related information that is recorded on secondary storage. • A sequence of bits, bytes, lines, or records whose meaning is defined by creator and user. • usually mapped to storage on a nonvolatile physical device by OS • 文件内形成一个：Contiguous logical address space

6. What is file? (cont.) • A file can be • Data file (numeric, character, binary); program file; … • A file may have a certain defined structure according to its type: • .txt; .c, .s, .S, .h, …; .o; .out, .exe, .elf, .com, … • How to describe a file • Attribute 文件属性 • Operation 文件操作 • Type 文件类型 • Structure文件结构

7. File Attributes文件属性 • Name • The only information kept in human-readable form • Type • needed for systems that support different types • Location • pointer to file location on device • Size • current file size

8. Protection • Access controls who can read, write, execute the file. • Time(s), date, and user identification • data for protection, security, and usage monitoring. • These information are kept in the directory structure, which is maintained on the disk.

9. 文件操作 • Basic file operations – minimal set • Create: file space + a directory entry • Write: write pointer • Read: read pointer • Seek: reposition within file • Delete: • Truncate • Requires read and write pointers, or a current-file-position pointer.

10. Combine these basic operations to perform other operations • Append: Seek to the end, and write • Copy: Create a new file, read old file and write into the new file • other • Rename • Get/set file attributes • Etc.

11. Opening a file • Open(filename1) finds the file and stores a pointer to it in an open file table (OFT). • 从目录中搜索filename1对应的目录项；将目录项中的内容拷贝到内存中；在OFT中保存指向目录项的指针；返回在OFT中的序号 • Open-file table • ID = index to the table • I/O operations use the pointer rather than the file name so there is no need to search for the file each time.

12. Multi-user Variation • 多个用户会同时使用同一个文件 • Use two levels of internal open-file tables: • Per-process table contains some process-dependent file information • E.g. Access rights, file pointers • System-wide table contains some process-independent file information • E.g. location on disk, size, open count, lock(s)

13. OFT process P1 OFT system OFT process P2 Two level open-file tables

14. Close a file • close (filename2) • Removes the content of the file from memory and removes its entry in the open-file table

15. Typical file types

16. 内容提要 • 文件定义、属性、操作和类型 • 文件的逻辑结构及其访问模式 • 文件系统的组织 • 目录结构 • 文件保护 • 文件系统的层次结构 • 外存分配算法和空闲空间的管理

17. 文件的逻辑结构 • 文件的物理结构 • 文件的逻辑结构 • 文件的逻辑结构： • 无结构，流式文件 • 有结构，记录式文件。在逻辑上，文件由若干个记录组成 • 有结构文件 • 记录：定长 vs. 不定长 • 记录的组织：顺序（按某种规则排序）、索引、索引顺序

18. 文件的访问模式 • 顺序访问：一个记录接着一个记录的访问 • 适用于磁带 • 文件读写指针渐变 • 直接访问（随机访问） • 根据要访问的记录的序号，计算出要访问的偏移 • 设置文件读写指针 • 索引访问 • 根据索引，查询要访问的偏移 • 设置文件读写指针

19. 内容提要 • 文件定义、属性、操作和类型 • 文件的逻辑结构及其访问模式 • 文件系统的组织 • 目录结构 • 文件保护 • 文件系统的层次结构 • 外存分配算法和空闲空间的管理

20. 文件系统的组织 • 分区：Partition (mini-disks, volumes)，可以是 • 一个磁盘 • 磁盘的一个部分provide separate logical spaces on one disk • 多个磁盘group several disks into a single logical space • Files + directories • Directory 目录 • holds file information (e.g. name, location, size, type) for all files in that partition

22. Directory Files F 1 F 2 F 3 F 4 F n 目录结构 • A collection of nodes containing information about all files. • A symbol table • Both the directory structure and the files reside on disk. • Backups of these two structures are kept on tapes.

23. Name Type Address Current length Maximum length Date last accessed (for archival) Date last updated (for dump) Owner ID (who pays) Protection information (discuss later) DOS FCB (file control block) 32 bytes each May cost many I/O operations to search for an entry UNIX Inode索引结点 Store most of file attributes in inode Directory entry contains file name and a pointer to the inode. 16 bytes Information in a Device Directory

24. Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system Search in the table for an entry Insert an entry Delete an entry Modify an entry … Operations Performed on Directory

25. Organize the Directory (Logically) to Obtain • Efficiency – locating a file quickly. • Naming – convenient to users. • Two users can have same name for different files. • The same file can have several different names. • Grouping – human convention • logical grouping of files by properties, (e.g., all Pascal programs, all games, …)

26. Directory Structures • Single-level directory（1级目录） • Two-level directory（2级目录） • Tree-structured directory（树型） • Acyclic-graph directory（无环图）

27. Single-Level Directory 1级目录 • Easy to support and understand. • A single directory for all • Very low searching speed, O(N)

28. Problems start when there are large numbers of files and/or users • Naming problem • Small naming space • Name collision

29. Two-Level Directory 2级目录 • Separate directory for each user • UFD, User File Directory • Each entry owns information for a user’s file • MFD, Master file directory • Each entry contains: User name, Pointer to his UFD

30. Can have the same file name for different user • Efficient searching • Easy management：Add/delete a user • Security VS. Sharing • MFD, system administrator • UFD, isolated from other users • How to share? • Directory tree (inverted tree) & path name • Search path • A UFD may be very large, then …

31. 树型目录结构

32. 树型目录结构(Cont.) • Root directory & directory & subdirectory • Regular file VS. subdirectory • Treat a subdirectory like another file • use a special bit in the directory entry to distinguish a file (0) from a subdirectory (1) • Absolute vs. relative path names? • e.g. /spell/words/rade ../spell/words/rade • Current directory (working/searching directory)

33. Acyclic-Graph Directories • 引入文件和目录的共享 • Two different names (aliasing)

34. Acyclic-Graph Directories (Cont.) • Implementation • Symbolic links 符号链接 • A special directory entry (link) • The content of such file is the path name of the real file/directory • How to traverse a directory contains symbolic links • Duplicates directory entries • Hard to maintain consistency

35. The problems existed • 遍历问题 • Different names, actual only one file • 删除问题 • Dangling pointer • Another implementation: hard link • File-reference list  reference count • How to ensure there are no cycles

36. Protection • Reliability可靠性 • Guarding against physical damage • File systems can be damaged by • Hardware problems, power surges or failures, head crashed, dirt, temperature extremes, or Vandalism • Generally provided by duplicate copies of files (disk  tape, …) • Protection 保护 • Guarding against improper access

37. Protection in multi-user system • Types of access • Read • Write • Execute • Append • Delete • List • File owner/creator should be able to control: • what can be done, by whom

38. Access Lists and Groups • make access dependent on the ID of the user • Access list • Associate with each file and directory an access list. • Access list specifies for each listed user name and the types of access allowed. • Stored in each directory entry • Length problem，Solution RWX a) owner access 7  1 1 1 RWX b) groups access 6  1 1 0 RWX c) public access 1  0 0 1

39. owner group public chmod 761 game Access lists and groups • Ask manager to create a group (unique name), say G, and add some users to the group. • For a particular file (say game) or subdirectory, define an appropriate access. • Attach a group to a file chgrpG game • 感受： • 在Linux中，使用 ls －la命令可以看到

40. File-System Structure • File • 逻辑存储单元 • Collection of related information • File system resides on secondary storage (disks) • 文件系统的组织 • How the file system should look to the user • How to map the logical file system onto the physical secondary-storage devices • File system organized into layers

41. Layered file system

42. 内容提要 • 文件定义、属性、操作和类型 • 文件的逻辑结构及其访问模式 • 文件系统的组织 • 目录结构 • 文件保护 • 文件系统的层次结构 • 磁盘调度算法、外存分配算法和空闲空间的管理

43. 磁盘调度算法 • 先来先服务 • 最短寻道时间优先 • 扫描算法SCAN • 循环扫描算法CSCAN • C-LOOK • 假设请求队列为： (0-199).98, 183, 37, 122, 14, 124, 65, 67起初磁头位置在53

44. 先来先服务

45. 最短寻道时间优先

46. 扫描算法SCAN

47. 循环扫描算法CSCAN

49. C-LOOK

50. 外存分配算法 • 连续分配 • 链接分配 • 隐式 VS. 显式 • 索引分配 • 单级索引分配、多级索引分配、混合分配方式 • Reading： • 计算机操作系统，汤子瀛，二版，9.2节或三版的6.3节 • Operating System Concepts, 7th Edition, section 11.4, P421-