Operating systems. Lecture 7 File Systems. Narcis ILISEI, oct 2014

Transcription

1 Operating systems Lecture 7 File Systems Narcis ILISEI, oct 2014

2 Goals for today File Systems FS Operations Inodes, directories Example FS: FAT, ISO9660 C++ FS access API Examples Exercises

3 File systems - FS FS = The way in which files are named and where they are placed logically for storage and retrieval. FS = a special-purpose database for the storage, organization, manipulation, and retrieval of data FS may use a data storage device such as a hard disk or CDROM and involve maintaining the physical location of the files

4 File Systems (1) Essential requirements for long-term information storage: It must be possible to store a very large amount of information. The information must survive the termination of the process using it. Multiple processes must be able to access the information concurrently.

5 File Systems (2) Think of a disk as a linear sequence of fixed-size blocks and supporting reading and writing of blocks. Questions that quickly arise: How do you find information? How do you keep one user from reading another s data? How do you know which blocks are free?

6 Example Program Using File System Calls (1)... Figure 4-5. A simple program to copy a file.

7 Example Program Using File System Calls (2) Figure 4-5. A simple program to copy a file.

8 Hierarchical Directory Systems (1) Figure 4-6. A single-level directory system containing four files.

9 Hierarchical Directory Systems (2) Figure 4-7. A hierarchical directory system.

10 Path Names Figure 4-8. A UNIX directory tree.

11 File Operations The most common system calls relating to files: Create Delete Open Close Read Write Append Seek Get Attributes Set Attributes Rename

12 Directory Operations System calls for managing directories: Create Delete Opendir Closedir Readdir Rename Link Uplink

13 Summary File Systems FS Operations Inodes, directories Example FS: FAT, ISO9660 C++ FS access API Examples

14 File System Layout Figure 4-9. A possible file system layout.

15 Contiguous Allocation Figure (a) Contiguous allocation of disk space for 7 files. (b) The state of the disk after files D and F have been removed.

16 Linked List Allocation Figure Storing a file as a linked list of disk blocks.

17 Linked List Allocation Using a Table in Memory Figure Linked list allocation using a file allocation table in main memory.

18 Inodes Data structure that holds information about FS objects like files, directories. When a file system is created, data structures are created that contain information about files. Each file is associated with an inode that is identified by an inode number ("i-number" or "inode") Inodes store information on files, such as user and group ownership, access mode (read, write, execute permissions) and type of file. The inode number indexes a table of inodes in a known location on the device; from the inode number, the kernel can access the contents of the inode, including the data pointers, and so the contents of the file. A file's inode number can be found using the ls -i command, while the ls -l command will retrieve inode information (i.e. the file information). On many types of file systems the number of inodes available is fixed at file system creation, limiting the maximum number of files the file system can hold. A typical fraction of space allocated for inodes in a file system is 1% of total size.

19 I-nodes Figure An example i-node.

20 Implementing Directories (1) Figure (a) A simple directory containing fixed-size entries with the disk addresses and attributes in the directory entry. (b) A directory in which each entry just refers to an i-node.

21 Implementing Directories (2) Figure Two ways of handling long file names in a directory. (a) In-line. (b) In a heap.

22 Shared Files (Links) Figure File system containing a shared file.

23 Shared Files (2) Figure (a) Situation prior to linking. (b) After the link is created. (c) After the original owner removes the file.

24 Links: Symbolic vs Hard links Hard links Directory entries that contain the same target inode Another pointer to the same file inode do not require any extra disk space, just a counter in the inode to keep track of how many there are. are restricted to pointing to files within their own partition. Symbolic links Just a regular file containing a kind of URL need space to store the name of the file pointed to. can point to other machines or over the Internet.

25 The Linux Ext2 File System (2) Figure (a) A Linux directory with three files. (b) The same directory after a voluminous file has been removed. Note that deleting a file reduces to reassinging the link to next file in directory inode.

26 The Linux Ext2 File System (3) Figure Some fields in the i-node structure in Linux

27 The Linux Ext2 File System (4) Figure The relation between the file descriptor table, the open file description table, and the i-node table.

28 MS Dos file system It finds the address of the first block in the directory entry. It then follows the chain of block pointers in the FAT until it has located the block it needs. It then remembers this block number for the next read system call.

29 The MS-DOS File System (1) Figure The MS-DOS directory entry.

30 The MS-DOS File System (2) Figure Maximum partition size for different block sizes. The empty boxes represent forbidden combinations.

31 Summary File Systems FS Operations Inodes, directories Allocation structure Example FS: FAT, ISO9660 C++ FS access API Examples

32 Allocation structures The most used ways of keeping track of free blocks are: Bitmaps Linked lists (see memory management)

33 Linked lists vs Bitmaps Figure (a) Storing the free list on a linked list. (b) A bitmap.

34 Disk Quotas Figure Quotas are kept track of on a per-user basis in a quota table.

35 Summary File Systems FS Operations Inodes, directories Allocation structure Example FS: FAT, ISO9660 C++ FS access API Examples

36 File systems for CDs and DVDs ISO 9660 Rockridge extension Joliet extension Note: all files/sizes are known in advance! No free blocks management is necessary

37 The ISO 9660 File System (CD, DVDs) Figure The ISO 9660 directory entry.

38 Joliet Extensions Joliet extension fields: Long file names. Unicode character set. Directory nesting deeper than eight levels. Directory names with extensions

39 Summary File Systems FS Operations Inodes, directories Allocation structure Example FS: FAT, ISO9660 C++ FS access API Examples

40 C vs C++ FS access API C API has a file handle as central data structure. Library functions work with the handle: Open, close, seek Write, read from / to buffers C++ API Has the concept of stream Streams can be chained Data type awareness via operator overloading

41 C++ File Streams / write ifstream - open the file for input ofstream - open the file for output fstream - open the file for input/output/both Writing to a file Declare an ofstream var. Open a file with it. Write to the file Close it. Other write methods

42 C++ File streams / read Reading from a file Declare an ifstream var. Open a file with it. Read from the file Close it. fstream supports both IN / OUT Other read methods

43 C++ File streams / attributes

44 Summary File Systems FS Operations Inodes, directories Allocation structure Example FS: FAT, ISO9660 C++ FS access API Examples