11Chapter 11: File-System Interface■File Concept■Access Methods■Directory Structure■File System Mounting■File Sharing ■ProtectionFile Concept■Contiguous logical address space■Types: ✦Data✔numeric✔character✔binary✦ProgramFile 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✦ProgramFile 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.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 Fiin memory to directory structure on disk.File Types – Name, Extension22Access Methods■Sequential Accessread nextwrite next resetnoread after last write(rewrite)■Direct Accessread nwrite nposition to nread nextwrite next rewrite nn= relative block numberSequential-access FileSimulation of Sequential Access on a Direct-access FileExample of Index and Relative FilesDirectory Structure■A collection of nodes containing information about all files.F 1F 2F 3F 4F nDirectoryFilesBoth the directory structure and the files reside on disk.Backups of these two structures are kept on tapes.A Typical File-system Organization33Information 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)Operations Performed on Directory■Search for a file■Create a file■Delete a file■List a directory■Rename a file■Traverse the file systemOrganize 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 Java programs, all games, …)Single-Level Directory■A single directory for all users.Naming problemGrouping problemTwo-Level Directory■Separate directory for each user.•Path name•Can have the same file name for different user•Efficient searching•No grouping capabilityTree-Structured Directories44Tree-Structured Directories (Cont.)■Efficient searching■Grouping Capability■Current directory (working directory)✦cd/spell/mail/prog✦typelistTree-Structured Directories (Cont.)■Absoluteor relativepath name■Creating a new file is done in current directory.■Delete a filerm<file-name>■Creating a new subdirectory is done in current directory.mkdir<dir-name>Example: if in current directory /mailmkdircountmailprog copy prt exp countDeleting “mail” ⇒deleting the entire subtree rooted by “mail”.Acyclic-Graph Directories■Have shared subdirectories and files.Acyclic-Graph Directories (Cont.)■Two different names (aliasing)■If dictdeletes 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.General Graph DirectoryGeneral 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 detectionalgorithm to determine whether it is OK.55File System Mounting■A file system must be mountedbefore it can be accessed.■A unmounted file system is mounted at a mount point.(a) Existing. (b) Unmounted PartitionMount PointFile Sharing■Sharing of files on multi-user systems is desirable.■Sharing may be done through a protectionscheme.■On distributed systems, files may be shared across a network.■Network File System (NFS) is a common distributed file-sharing method.Protection■File owner/creator should be able to control:✦what can be done✦by whom■Types of access✦Read✦Write✦Execute✦Append✦Delete✦ListAccess Lists and Groups■Mode of access: read, write, execute■Three classes of usersRWXa) owner access7⇒1 1 1RWXb) group access6⇒1 1 0RWXc) public access1⇒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 publicchmod 761 gameAttach a group to a filechgrp G