Chapter 7- File-System

1. Chapter 7-File-System • File Concept • Access Methods • Directory Structure • Protection • File-System Structure • Allocation Methods • Free-Space Management • Directory Implementation • Efficiency and Performance • Recovery Operating Systems

2. File Concept • Contiguous logical address space • Types: • Data • numeric • character • binary • Program Operating Systems

3. File Structure • None - sequence of words, bytes • Simple record structure • Lines • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file • Can simulate last two with first method by inserting appropriate control characters. • Who decides: • Operating system • Program Operating Systems

4. File Attributes • Name – 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. • Protection – controls who can do reading, writing, executing. • Time, date, and user identification – data for protection, security, and usage monitoring. • Information about files are kept in the directory structure, which is maintained on the disk. • Etc.. Operating Systems

5. File Operations • create • write • read • reposition within file – file seek • delete • truncate • open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory. • close (Fi) – move the content of entry Fi in memory to directory structure on disk. Operating Systems

6. File Types – name, extension Operating Systems

7. Access Methods • Sequential Access read next write next reset no read after last write (rewrite) • Direct Access read n write n position to n read next write next rewrite n n = relative block number Operating Systems

8. Tree-Structured Directories Operating Systems

9. Acyclic-Graph Directories • Have shared subdirectories and files. Operating Systems

10. General Graph Directory Operating Systems

11. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List Operating Systems

12. Access Lists and Groups • Mode of access: read, write, execute • Three classes of users RWX a) owner access 7  1 1 1 RWX b) groups access 6  1 1 0 RWX c) public access 1  0 0 1 • 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. owner group public chmod 761 game • Attach a group to a file chgrpG game Operating Systems

13. Allocation: Contiguous • Each file occupies a set of contiguous blocks on the disk. • Simple – only starting location (block #) and length (number of blocks) are required. • Random access. • Wasteful of space (dynamic storage-allocation problem). • Files cannot grow. • Mapping from logical to physical. Q LA/512 R • Block to be accessed = ! + starting address • Displacement into block = R Operating Systems

14. Allocation: Linked list • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. Operating Systems

15. Linked Allocation (Cont.) • Simple – need only starting address • Free-space management system – no waste of space • No random access • Mapping Q LA/511 R • Block to be accessed is the Qth block in the linked chain of blocks representing the file. • Displacement into block = R + 1 • File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2. Operating Systems

16. Allocation: Indexed Operating Systems

17. Indexed Allocation (Cont.) • Need index table • Random access • Dynamic access without external fragmentation, but have overhead of index block. • Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 words. We need only 1 block for index table. Q LA/512 R • Q = displacement into index table • R = displacement into block Operating Systems

18. Indexed Allocation – Mapping (Cont.) • Mapping from logical to physical in a file of unbounded length (block size of 512 words). • Linked scheme – Link blocks of index table (no limit on size). Q1 LA / (512 x 511) R1 • Q1= block of index table • R1is used as follows: Q2 R1 / 512 R2 • Q2= displacement into block of index table • R2displacement into block of file: Operating Systems

19. Indexed Allocation – Mapping (Cont.) • Two-level index (maximum file size is 5123) Q1 LA / (512 x 512) R1 • Q1= displacement into outer-index • R1is used as follows: Q2 R1 / 512 R2 • Q2= displacement into block of index table • R2displacement into block of file: Operating Systems

20. Combined Scheme: UNIX (4K bytes per block) Operating Systems

21. Free-Space Management 0 1 2 n-1 • Bit vector (n blocks) … 0  block[i] free 1  block[i] occupied bit[i] =  • Bit map requires extra space. Example: block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) • Easy to get contiguous files Operating Systems

22. MS-DOS scheme • FAT-File Access Table contains indices of the blocks allocated to all the files. • VTOC: Volume Table of Content is used to contain the highest level directory content, in the form of the file name and the index of its first block in the FAT • Each entry of a directory contains attributes such as file name, extension, attributes, time, date, size, and the first block number. • CP/M had one directory for all the files, where each of this directory entry contained all the attributes, with all allocated blocs listed in the same entry • Directory implementation: • Linear list of file names with pointer to the data blocks. • Hash Table – linear list with hash data structure. Operating Systems

23. File System Reliability • Bad block management • hw solution; allocate a special sector to hold bad blocka and spare blocks to replace them • sw solution: keep a list of bad blocks in a file and remove them from the list of free blocks • File system reliability • caused by system crash before modified data is written to the disk • there must be an effective utility program, such as fsck in UNIX, to reestablish the inconsistencies • a consistency check involves • a block is allocated to more than one file • a block appears more than once in the free list • file attributes, such as various counters are consistent Operating Systems

24. Efficiency and Performance • Factors effecting efficiency • disk allocation and directory algorithms • types of data kept in file’s directory entry • frequency of refreshment (flushing memory copies of blocks) • Performance increasing measures • disk cache – separate section of main memory for frequently sued blocks • free-behind and read-ahead – techniques to optimize sequential access • improve performance by dedicating section of memory as virtual disk, or RAM disk. • Use virtual memory for file I/O • use page replacement like algorithm in flushing/reclaiming the data blocks in use: order the blocks to be flushed in LRU order! Operating Systems

25. Recovery • Consistency checker – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies. • Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape). • Recover lost file or disk by restoring data from backup. Operating Systems