CSC 414 Chapter 11, Spring 2008

11.1 Applied Operating System Concepts Module 11: File-System Interface • File Concept • Access Methods • Directory Structure • Protection • File-System Structure • Allocation Methods • Free-Space Management • Directory Implementation • Efficiency and Performance • Recovery 11.2 Applied Operating System Concepts File Concept • Contiguous logical address space • Types: – Data  numeric  character  binary – Program 11.3 Applied Operating System Concepts 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 11.4 Applied Operating System Concepts 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. 11.5 Applied Operating System Concepts 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. 11.6 Applied Operating System Concepts File Types – name, extension Executable exe, com, bin or none ready-to-run machine- language program Object obj, o complied, machine language, not linked Source code c, p, pas, 177, asm, a source code in various languages Batch bat, sh commands to the command interpreter Text txt, doc textual data documents Word processor wp, tex, rrf, etc. various word-processor formats Library lib, a libraries of routines Print or view ps, dvi, gif ASCII or binary file Archive arc, zip, tar related files grouped into one file, sometimes compressed. File Type Usual extension Function 11.7 Applied Operating System Concepts 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 11.8 Applied Operating System Concepts Directory Structure • A collection of nodes containing information about all files. F 1 F 2 F 3 F 4 F n Directory Files • Both the directory structure and the files reside on disk. • Backups of these two structures are kept on tapes. 11.9 Applied Operating System Concepts Information in a Device Directory • 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) 11.10 Applied Operating System Concepts Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system 11.11 Applied Operating System Concepts 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 – logical grouping of files by properties, (e.g., all Pascal programs, all games, …) 11.12 Applied Operating System Concepts Single-Level Directory • A single directory for all users. • Naming problem • Grouping problem 11.13 Applied Operating System Concepts Two-Level Directory • Separate directory for each user. • Path name • Can have the saem file name for different user • Efficient searching • No grouping capability 11.14 Applied Operating System Concepts Tree-Structured Directories 11.15 Applied Operating System Concepts Tree-Structured Directories (Cont.) • Efficient searching • Grouping Capability • Current directory (working directory) – cd /spell/mail/prog – type list 11.16 Applied Operating System Concepts Tree-Structured Directories (Cont.) • Absolute or relative path name • Creating a new file is done in current directory. • Delete a file rm <file-name> • Creating a new subdirectory is done in current directory. mkdir <dir-name> Example: if in current directory /spell/mail mkdir count mail prog copy prt exp count • Deleting “mail”  deleting the entire subtree rooted by “mail”. 11.17 Applied Operating System Concepts Acyclic-Graph Directories • Have shared subdirectories and files. 11.18 Applied Operating System Concepts Acyclic-Graph Directories (Cont.) • Two different names (aliasing) • If dict deletes list  dangling pointer. Solutions: – Backpointers, so we can delete all pointers. Variable size records a problem. – Backpointers using a daisy chain organization. – Entry-hold-count solution. 11.19 Applied Operating System Concepts General Graph Directory 11.20 Applied Operating System Concepts General Graph Directory (Cont.) • How do we guarantee no cycles? – Allow only links to file not subdirectories. – Garbage collection. – Every time a new link is added use a cycle detection algorithm to determine whether it is OK. 11.21 Applied Operating System Concepts Protection • File owner/creator should be able to control: – what can be done – by whom • Types of access – Read – Write – Execute – Append – Delete – List 11.22 Applied Operating System Concepts 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 chgrp G game 11.23 Applied Operating System Concepts File-System Structure • File structure – Logical storage unit – Collection of related information • File system resides on secondary storage (disks). • File system organized into layers. • File control block – storage structure consisting of information about a file. 11.24 Applied Operating System Concepts Contiguous Allocation • 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. LA/512 Q R – Block to be accessed = ! + starting address – Displacement into block = R 11.25 Applied Operating System Concepts Linked Allocation • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block = 11.26 Applied Operating System Concepts • Allocate as needed, link together; e.g., file starts at block 9 11.27 Applied Operating System Concepts Linked Allocation (Cont.) • Simple – need only starting address • Free-space management system – no waste of space • No random access • Mapping – 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. LA/511 Q R 11.28 Applied Operating System Concepts Indexed Allocation • Brings all pointers together into the index block. • Logical view. index table 11.29 Applied Operating System Concepts Example of Indexed Allocation 11.30 Applied Operating System Concepts 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. LA/512 Q R – Q = displacement into index table – R = displacement into block 11.31 Applied Operating System Concepts 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). LA / (512 x 511) Q1 R1 – Q1 = block of index table – R1 is used as follows: R1 / 512 Q2 R2 – Q2 = displacement into block of index table – R2 displacement into block of file: 11.32 Applied Operating System Concepts Indexed Allocation – Mapping (Cont.) • Two-level index (maximum file size is 5123) LA / (512 x 512) Q1 R1 – Q1 = displacement into outer-index – R1 is used as follows: R1 / 512 Q2 R2 – Q2 = displacement into block of index table – R2 displacement into block of file: 11.33 Applied Operating System Concepts Indexed Allocation – Mapping (Cont.)  outer-index index table file 11.34 Applied Operating System Concepts Combined Scheme: UNIX (4K bytes per block) 11.35 Applied Operating System Concepts Free-Space Management • Bit vector (n blocks) … 0 1 2 n-1 bit[i] =  0  block[i] free 1  block[i] occupied • Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit 11.36 Applied Operating System Concepts Free-Space Management (Cont.) • 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 • Linked list (free list) – Cannot get contiguous space easily – No waste of space • Grouping • Counting 11.37 Applied Operating System Concepts Free-Space Management (Cont.) • Need to protect: – Pointer to free list – Bit map  Must be kept on disk  Copy in memory and disk may differ.  Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk. – Solution:  Set bit[i] = 1 in disk.  Allocate block[i]  Set bit[i] = 1 in memory 11.38 Applied Operating System Concepts Directory Implementation • Linear list of file names with pointer to the data blocks. – simple to program – time-consuming to execute • Hash Table – linear list with hash data structure. – decreases directory search time – collisions – situations where two file names hash to the same location – fixed size 11.39 Applied Operating System Concepts Efficiency and Performance • Efficiency dependent on: – disk allocation and directory algorithms – types of data kept in file’s directory entry • Performance – disk cache – separate section of main memory for frequently sued blocks – free-behind and read-ahead – techniques to optimize sequential access – improve PC performance by dedicating section of memroy as virtual disk, or RAM disk. 11.40 Applied Operating System Concepts Various Disk-Caching Locations 11.41 Applied Operating System Concepts 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.