Silberschatz, Galvin and Gagne  Operating System Concepts Chapter 11: File-System Interface File Concept Access Methods Directory Structure File System Mounting File Sharing Protection

Silberschatz, Galvin and Gagne  Operating System Concepts File Concept Contiguous logical address space Types:  Data  numeric  character  binary  Program

Silberschatz, Galvin and Gagne  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

Silberschatz, Galvin and Gagne  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.

Silberschatz, Galvin and Gagne  Operating System Concepts File Types – Name, Extension

Silberschatz, Galvin and Gagne  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

Silberschatz, Galvin and Gagne  Operating System Concepts Sequential-access File

Silberschatz, Galvin and Gagne  Operating System Concepts Simulation of Sequential Access on a Direct-access File

Silberschatz, Galvin and Gagne  Operating System Concepts Example of Index and Relative Files

Silberschatz, Galvin and Gagne  Operating System Concepts Directory Structure Files in disks are organized in the following ways:  Disks are split into partitions (minidisks, volumes).  A device directory or volume table of contents records files information in each partition.

Silberschatz, Galvin and Gagne  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.

Silberschatz, Galvin and Gagne  Operating System Concepts A Typical File-system Organization

Silberschatz, Galvin and Gagne  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)

Silberschatz, Galvin and Gagne  Operating System Concepts Operations Performed on Directory Search for a file Create a file: New files need to be added to the directory Delete a file List a directory Rename a file: Renaming a file may also allow its position within the directory structure to be changed. Traverse the file system

Silberschatz, Galvin and Gagne  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 Java programs, all games, …)

Silberschatz, Galvin and Gagne  Operating System Concepts Single-Level Directory A single directory for all users. Naming problem Grouping problem

Silberschatz, Galvin and Gagne  Operating System Concepts Two-Level Directory Separate directory for each user (user file directory – UFD). System has the master file directory (MFD).  Path name – A user and file name define it.  Can have the same file name for different user  The sequence of directories searched when a file is named is called the search path.  Efficient searching  No grouping capability

Silberschatz, Galvin and Gagne  Operating System Concepts Tree-Structured Directories Properties of tree-structured directories:  A tree is the most common directory structure.  The tree has a root directory.  Every file has a unique path name.  A directory (or subdirectory) contains a set of files or subdirectories. The current directory should contain most of the files that are of current interest to the user. Efficient searching Grouping Capability Current directory (working directory)  cd /spell/mail/prog  type list

Silberschatz, Galvin and Gagne  Operating System Concepts Tree-Structured Directories

Silberschatz, Galvin and Gagne  Operating System Concepts Tree-Structured Directories (Cont.) Absolute or relative path name  An absolute path name begins at the root.  A relative path name defines a path from the current directory.  Some systems also allow users to define their own search paths. 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”.

Silberschatz, Galvin and Gagne  Operating System Concepts Acyclic-Graph Directories A tree structure prohibits the sharing of files or directories. An acyclic graph (a generalization of the tree-structured) allows directories to have shared subdirectories and files.

Silberschatz, Galvin and Gagne  Operating System Concepts Acyclic-Graph Directories (Cont.) Two different names (aliasing) point to the same file. If dict deletes count  dangling pointer for spell/count. Solutions:  Symbolic links  The file is deleted until all references to it are deleted.  Entry-hold-count solution: hard links in UNIX

Silberschatz, Galvin and Gagne  Operating System Concepts General Graph Directory

Silberschatz, Galvin and Gagne  Operating System Concepts General Graph Directory (Cont.) If cycles are allowed to exist in the directory, repeated search could happen and the reference count may be nonzero (Figure 11.10). 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.

Silberschatz, Galvin and Gagne  Operating System Concepts File System Mounting A file can be opened. A file system must be mounted before it can be accessed. Mount procedure:  Mount device to the location at which to attach the file system (mount point).  Verify the device contains a valid file system.  Note that a file system is mounted at the specified mount point. A unmounted file system (I.e. Fig (b)) is mounted at a mount point. The Macintosh OS automatically mounts the floppy disk. The Microsoft Windows mount devices at boot time. In UNIX, the mount commands are explicit.

Silberschatz, Galvin and Gagne  Operating System Concepts (a) Existing. (b) Unmounted Partition

Silberschatz, Galvin and Gagne  Operating System Concepts Mount Point

Silberschatz, Galvin and Gagne  Operating System Concepts File Sharing Sharing of files on multi-user systems is desirable. Most systems implement owner attributes by managing a list of user names and associated user identifiers (UIDs) and group identifiers (GIDs). Sharing may be done through a protection scheme. On distributed systems, files may be shared across a network. Network File System (NFS) is a common distributed file- sharing method.

Silberschatz, Galvin and Gagne  Operating System Concepts Remote File System Networking allows the sharing of resources:  User can manually transfer files. ftp with anonymous access.  Distributed file system (DFS)  World Wide Web To ease the management of client-server services, distributed information system, or distributed naming services can be used. Domain name system (DNS) provides host-name-to- network-address translations for the entire Internet. Sun Microsystems use yellow page (network information service (NIS)). The industry is moving toward lightweight directory- access protocol (LDAP) as a secure, distributed naming mechanism.

Silberschatz, Galvin and Gagne  Operating System Concepts File Sharing Redundant arrays of inexpensive disks (RAID) can prevent the loss of a disk from resulting in the loss of data. Consistency semantics are directly related to the process synchronization algorithms. The UNIX file system uses the following semantics:  Writes to an open file by a user are visible immediately to other users.  A file has a single image interleaving all accesses. The session semantics enforces no constraints on scheduling accesses. An immutable shared file is a file declared as shared, it cannot be modified.

Silberschatz, Galvin and Gagne  Operating System Concepts Protection When information is kept in a computer system, we want to keep it safe from physical damage (reliability) and improper access (protection). File owner/creator should be able to control:  what can be done  by whom Types of access  Read  Write  Execute  Append  Delete  List The most general access control is done by an access-control list (ACL), which specifies the user name and the types of access allowed for each users.

Silberschatz, Galvin and Gagne  Operating System Concepts 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

Silberschatz, Galvin and Gagne  Operating System Concepts Other Protection Approaches / UNIX Associate a password with each file. In UNIX, directory protection is handled similarly to file protection.