Chapter 10: File-System Interface

1. Chapter 10: File-System Interface • What is a file-system? • An abstract representation of physical devices capable of storing and retrieving binary information (files) • consists of partitions (volumes (NT), minidisks, etc), directories, and files • “virtual” file-systems may be smaller or greater in size then the physical devices used to store the information • What is a file? • A file is an abstract data type, defined by the operations that can be performed on them • A named collection of related information recorded on a secondary storage (non-volatile) device • Uniform logical view of information storage • A logical storage unit (mapped by the OS onto a physical device) • Who decides how the meaning is encoded (structure)? • Operating system • Program CEG 433/633 - Operating Systems I

2. File Attributes • A file is a sequence of bits, bytes, lines or records whose format encodes meaning as defined by the file’s creator or user: • “Freeform” structure: sequence of words, bytes • Simple regular structure: fixed or variable length records • Complex structures: formatted document, relocatable load file • Filetypes: • Data (numeric, character, binary) • Executable (loader can bring into memory and execute) • Directory (keeps information about other files) • 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. CEG 433/633 - Operating Systems I

3. File Types – name, extension CEG 433/633 - Operating Systems I

4. 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 CEG 433/633 - Operating Systems I

5. File Operations • Files are an abstract data type, defined by the operations that can be performed on the. The OS must provide system calls for the following types of operations: • create: find space, write to file, update directory • write: name and position, update size, update file pointer • read: name and position, update file pointer • seek: reposition within file, update file pointer • delete: deallocate space, erase directory entry • 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. • Files must be explicitly opened via open() to create a entry in the open-file table. The index to the entry in the open-file table is the “file handle”. • # of processes w/ current-file position pointer (offset) for each • Information regarding physical location of file on disk CEG 433/633 - Operating Systems I

6. F 1 F 2 F 3 F 4 F n Directory Structure • A collection of nodes containing information about all files. • Operations: • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system Directory Files • Both the directory structure and the files reside on disk. • Backups of these two structures are kept on tapes. CEG 433/633 - Operating Systems I

7. Single-Level Directory • A single directory for all files (all users). • Naming problem • Each file must have a unique name - • it may be difficult to keep track of “used” names • What if two users want to use the same filename? • What if two users have the _same_ file under two different names? • Grouping problem • files are not grouped by logical properties (C programs, games, etc.) • Efficiency – how can you locate a file quickly (if you don’t know the name…) CEG 433/633 - Operating Systems I

8. Two-Level Directory • Separate directory for each user. • Path name • Can have the same file name for different user • Efficient searching • No grouping capability CEG 433/633 - Operating Systems I

9. Tree-Structured Directories • Efficient searching • Grouping Capability • Current directory (working directory) • cd /spell/mail/prog • type list CEG 433/633 - Operating Systems I

10. 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”. CEG 433/633 - Operating Systems I

11. Acyclic-Graph Directories • Have shared subdirectories and files. • Different names (aliasing) • Solutions: • Backpointers, so we can delete all pointers.Variable size records a problem. • Backpointers using a daisy chain organization. • Entry-hold-count solution. CEG 433/633 - Operating Systems I

12. General Graph Directory • 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. CEG 433/633 - Operating Systems I

13. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List • Access Lists • Access Control Lists (ACLs) CEG 433/633 - Operating Systems I

14. Access Lists and Groups in UNIX • 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 chgrpG game CEG 433/633 - Operating Systems I