14.1 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 10 & 11: File-System Interface and Implementation

14.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition

14.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical File-system Organization

14.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Files Contiguous logical address space Types: Data: numeric, character, binary Program Structure None - sequence of words, bytes Simple record structure  Lines  Fixed length  Variable length Complex Structures  Formatted document  Relocatable load file Who decides the structure: Operating system Program

14.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system 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

14.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Operations File is an abstract data type Create Write Read Reposition within file Delete Truncate Open(F i ) – search the directory structure on disk for entry F i, and move the content of entry to memory Close (F i ) – move the content of entry F i in memory to directory structure on disk

14.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Open Files Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last process closes it Disk location of the file: cache of data access information Access rights: per-process access mode information Open file locking Provided by some operating systems and file systems Mediates access to a file Mandatory or advisory:  Mandatory – access is denied depending on locks held and requested  Advisory – processes can find status of locks and decide what to do

14.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Types – Name, Extension

14.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 (relative to the beginning of the file)

14.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

14.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

14.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Single-Level Directory A single directory for all users Naming problem Grouping problem

14.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability

14.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories Efficient searching Grouping capability Current directory (working directory)

14.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm Creating a new subdirectory is done in current directory mkdir Example: if in current directory /mail mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

14.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition General Graph Directory Links to share files – only one copy exists How do we guarantee no cycles? Allow only links to files not subdirectories Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK

14.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Sharing Sharing of files on multi-user systems is desirable Sharing may be done through a protection scheme User IDs identify users, allowing permissions and protections to be per-user Group IDs allow users to be in groups, permitting group access rights On distributed systems, files may be shared across a network Network File System (NFS) is a common distributed file-sharing method

14.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List

14.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7  RWX b) group access 6  RWX c) public access1  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. ownergrouppublic chmod761game Attach a group to a file chgrp G game

14.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Sample UNIX Directory Listing

14.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Moving-head Disk Mechanism Disk access costs: Transfer time + access time Access time: –Seek time: time to move the disk heads to the desired cylinder –Rotational latency: time for rotating to desired sector

14.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Files: Contiguous allocation (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and E have been removed

14.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Files: Linked List The links are on the disk (using block numbers)

14.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked List: File Allocation Table Keep the list links in a separate table in memory

14.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition i-nodes

14.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A UNIX i-node Max file size: d direct pointers, n indirect pointers p/block Blocksize * (d + n + n 2 + n 3 )

14.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Consider the organization of a UNIX file as represented by the inode. Assume there are 12 direct block pointers and a singly, doubly and triply indirect pointer in each inode. Further, assume that the system block size and the disk sector size are both 8K. If the disk block pointer is 32 bits, with 8 bits to identify the physical disk and 24 bits to identify the physical block, then: What is the maximum file size supported by this system? What is the maximum file system partition supported by this system? Assuming no information other than the file inode is already in main memory, how many disk accesses are required to access the byte in position 100K+10, where K=1024?

14.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A UNIX V7 directory entry

14.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Reminder: Disk organization in UNIX

14.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The steps in looking up /usr/ast/mbox

14.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Disk Space Management (a) Storing the free list on a linked list (b) A bit map