Chapter 10: File System Interface

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1 Chapter 10: File System Interface File Concept Access Methods Directory Structure File System Mounting File Sharing Protection Silberschatz et al, System Concepts 7/e, Wiley, 2005 File System Definition The file system is the most user visible aspect of an operating system. It provides the mechanism for on-line storage of and access to both data and programs of the OS and all the users of the computer system. The file system consists of two distinct parts: A collection of files, each storing related data, and a directory structure, which organizes and provides information about all the files in the system.

2 File Concept The file manager (FM) is a librarian and books are files (locate, allocate, store, deallocate, efficient access) A file = group of related records (flat file, no dimension!) A record = group of related fields A field = group of related bytes Types of files: Data files: might be: numeric character binary Program files: contains instructions File Types Name, Extension Pdf, gsview, gscript rar

3 File Structure File Concept (cont.) Simple record structure Fixed length (easiest to access but create leftovers!) Variable length (no leftovers, hard to access!) An executable file have sometimes a specific structure that helps the operating system to load the file in the memory (where to load in memory an executable file and what is the address of the first instruction) The OS needs to have different codes to support multiple file structures: disadvantage DEC-VMS OS has a file system that supports three defined file structures, UNIX only 1(8bit-bytes) Internal File Structure In UNIX, a file is a stream of bytes, the logical record is 1 byte Physical record: disk unit = block, its size is defined by the sector size Packing technique (512 bytes/block) some portion of the last block of each file is wasted Internal fragmentation File Concept (cont.) 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

4 File Concept (cont.) Our interaction with the file Manager (embedded in user s program or submitted interactively by the user) Create Write Read Reposition within file file seek 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. Access Methods Sequential Access (editors, compilers) read next record (one record after the other) write next record reset (rewrite) Direct Access (based on a disk model and hashing) read n write n position to n read next write next rewrite n n = relative record (or block) number Airline-reservation system (access with key=flight number= block number) or use hash functions

5 Sequential-access File Other Access Methods based on Building an Index Table for the File

6 Directory Structure A collection of nodes containing information about all files. Directory Files F 1 F 2 F 3 F 4 F n Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes. A Typical File-system Organization 2 partitions in one disk 2 disks in one partition

7 Each partition contains information about files within it in entries 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 system (save the contents and structure of the entire file system (backup))

8 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, ) Single-Level Directory users A single directory for all users. Naming problem! All these files are in the same directory Grouping problem!

9 Two-Level Directory (tree height=2) Separate directory for each user. Path name Can have the same file name for different user Efficient searching No groupings (we have only 2 levels!) No cooperation (no access of one another s file!) Tree-Structured Directories time

10 Tree-Structured Directories (Cont.) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog/obj type time (time is a file) 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 /mail mkdir count mail prog copy prt exp count Deleting mail deleting the entire subtree rooted by mail.

11 Acyclic-Graph Directories (graph with no cycles!) Have shared subdirectories and files (programmers working in a joint project). shared file There are no cycles in this graph! list Acyclic-Graph Directories (Cont.) Two different names for a same file (aliasing) If dict deletes list dangling pointer. Solutions: Backpointers, so we can delete all pointers (or delete symbolic links if they describe the sharing concept: expensive search: 1 file many links) Symbolic links are left to the users who should notice that the file is deleted in the case of Unix or Windows (with its all flavors!) File deletion is done when reference count = 0 (no reference to the file anymore!): a new link or directory entry increments the reference count and a link deletion decrements it.

12 General Graph Directory (came from a treestructured directory by adding links: it may contain cycles!) How do we guarantee no cycles? Allow only links to files not to subdirectories Garbage collection (involves traversing the entire file system, marking everything that can be accessed) Every time a new link is added use a cycle detection algorithm to determine whether it is OK. File System Mounting A file system must be mounted before it can be accessed to processes on the system. A unmounted file system (i.e. Fig (b)) is mounted at a mount point (empty directory).

13 (a) Existing. Files are accessed (b) Unmounted Partition Files are not accessed Partition residing in /device/dsk Mount Point (during boot time or manually) Mounting of the partition residing on /dev/dsk over /users

14 File Sharing Sharing of files on multi-user systems is desirable. 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. Protection File owner/creator should be able to control: what can be done by whom Main types of access Read Write Execute

15 Protection (cont.) Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access RWX b) group access RWX c) public access Ask system 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 total access : owner group public Attach a group to a file chgrp G game chmod 761 game