File-System Interface

Outline of this lecture

In this lecture, we will discuss the following:

• File Concept
• Access Methods
• Directory Structure
• Protection

File-System (FS) Structure

• File system consists of:
  • A collection of files – storing related data
  • Directory structure- provides information about all the files in the system.
• File system resides on secondary storage (disks).

File Concept

• A collection of data- normally is stored on a secondary storage device.
• File types refer to classifying the content of the file, such as:

 Types:



Data

numeric



character



binary





Program

 Contents defined by file ’ s creator

Many types



Consider text file, source file, executable file



File Attributes

A file has several attribute- vary from one O/S to another but consist of these:

• 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 – this information can be useful for protection, security, and usage monitoring.
• Information about files are kept in the directory structure, which is maintained on the disk.

An OS must provide a number of operations associated with files so that users can safely store and retrieve data. Typical 6 basic operations are:

• Create - find space in the file system & entry for new file must be made in dir.
• Write - make system call specifying both the name of the file & inf to be written to it.
• Read - use system call specifying the file’s name & where next block should be put.
• Seek -repositioning within file (searching the directory for the appropriate entry).
• Delete - search the directory for the named file, erase the directory entry.
• Truncate - erase the contents of file but keep its attributes.
• Open(F) – search the directory structure on disk for entry F, and move the content of i i entry to memory.
• Close (F) – move the content of entry F in memory to directory structure on disk. i i

• Most of the file operations involve searching the directory for the entry associated with the named file. To avoid searching, the OS keeps a table
• Several pieces of data are needed to manage open files:
  • Open-file table: contains info abt all open files
  • File pointer: pointer must track the last read/write location as a current file position pointer, per process that has the file open
  • File-open count: counter of number of times a file is open by many processes, when the open count reaches 0, the file is no longer in use, and the file’s entry is removed from the open file table
  • Disk location of the file: most file operations require the system to modify data within the file. The info needed to locate the file on a disk is kept in memory
  • Access rights: each process opens a file in an access mode. This info is stored on the per-process table so the OS can allow/deny I/O requests.

• Files must conforms to a required structure understood by the OS. i.e. OS requires that the exe file have a specific structure, so that it can determine where in memory to load the file & what the location of the 1 st instruction.
• 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

Methods

Information stored in files must be accessed & read into memory.

So how can this information be accessed?

• Sequential Access
  • information in the file is processed in order- 1 record after another
  • Most common, Editors & compilers access files in this fashion.
• Direct Access / Relative Access
  • File is made up of fixed-length logical records, allow programs to read/write records rapidly in no particular order.
  • file is viewed as a numbered sequence of records.
  • Records are organized in a manner that allows direct access to a particular record without having to read any of the preceding records.
  • Allows blocks to be read or written in any order- no restriction on the order of reading/writing for a direct-access file.
  • Useful for immediate access to large amounts of info (DB).

• Can be built on top of direct Access
• General involve creation of an index for the file
• Index file - stores an index into another file
• With large files, the index file become too large -------- → solution
• Primary, secondary index file - primary index file contains pointers to the secondary one, which points to the actual data items.
• IBM indexed sequential-access method (ISAM)
  • Small master index, points to disk blocks of secondary index
  • File kept sorted on a defined key
  • All done by the OS
• VMS operating system provides index and relative files as another example (see next slide)

Operations Performed on Directory

• Search for a file - be able to search a directory structure to find particular file
• Create a file - new files need to be created & added to the directory.
• Delete a file- be able to remove a file from the directory.
• List a directory- be able to list the files in a directory.
• Rename a file- be able to change the name of a file when the contents or use of the file changes.
• Traverse the file system- be able to access every directory & every file within a directory structure.

• 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, ...)

• A tree has a root directory, & every file in the system has a unique path name.
• path name- root --- → subdirectories ---- specified file. →
• Efficient searching
• Grouping Capability: Each directory contains a set of files or Subdirectories.
• Current directory - contains most the files that are of current interest to the user.
• Absolute path name: begins at the root -- → a path down-- specified file. →
• Relative path name: defines path from the current directory.
• Deletion of a directory
  • If empty- can be deleted.
  • If not empty- some systems, will not delete a directory. e.g: MS-DOS. - other systems, remove the entire dir structure. e.g. UNIX rm <file-name>
    • But, if the command is issued in error-→ large no of files & directories will need to be restored.

• There are several ways to implement shared files and directories:
  • Link- pointer to another file or subdirectory (maybe implemented as relative/absolute path name).
  • Resolve the link – using path name to locate the real file
  • Duplicate File Information - Duplicating all information about the shared files in both sharing directories.
• Aliasing problem - A file can have multiple file absolute path names
• Deletion Problem: dangling pointers- remove the file whenever anyone deletes it
  • Solution
    • Delete only the link.
    • Preserve file until all references to it are deleted- when the file-reference list is empty, the file is deleted.

• When a file can be deleted?
• A value of 0 in the reference count → no reference → delete in acyclic graph
• When cycles exist, reference count may not be 0 even when it is no longer possible to reference to a file/directory, due to self-referencing/ cycle in the directory
  • Allow only links to file not subdirectories.
  • Garbage collection to determine when the last reference has been deleted & the disk space can be reallocated.
  • Every time a new link is added use a cycle detection algorithm to determine whether it is OK. Will the new link complete a cycle or not?

• Protection is needed so that users do not accidentally/deliberately destroy someone else’s information.
• Information is private & should not be read by other users.
• OS must provide means to protect the info in the file system.
• Many types of operations could be controlled:
  • Read
  • Write
  • Execute
  • Append
  • Delete
  • List