Module 4.0: File Systems

1
Module 4.0 File Systems

• File is a contiguous logical address space (of
  related records)
• Access Methods
• Directory Structure
• Protection
• File System implementation
• File-system structure
• Allocation Methods
• Free-space Management
• Directory Implementation
• Recovery, Efficiency, and Performance

2
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.

3
File Operations

• create
• write
• read
• reposition within file file seek
• delete
• truncate
• open(Fi) search the directory
  structure/descriptor 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/descriptor on disk.

4
File Types name, extension
5
File Structure of Data within Data Blocks

• None - sequence of words, bytes
• Simple record structure
• Lines
• Fixed length
• Variable length
• Tree of fixed or variable records with a key for
  each record.
• Who decides
• Operating system
• Program

6
Access Methods

• Sequential Access Tape Model. Reading records
  in order.
• read next
• write next
• reset
• Direct or Random Access Disk Model. Reading
  records in order and out of order.
• read n
• write n
• position to n
• read next
• write next
• Most files in modern OS are direct access.

7
Memory-Mapped Files

• Some OS provides map and unmap system calls.
• map will map the file into the address space at
  the virtual address. System calls like open,
  read, seek are not applicable.
• Eliminates the need for I/O, thus making it
  easier to program.
• Problems
• Hard to know the size of the file in advance
• Conflicts arise when sharing files between
  processes
• Example one process uses map and another uses
  open, read, write.
• How to deal with huge files that are bigger than
  a segment or the whole virtual address space.

8
Directory Structure

• A collection of nodes containing information
  about all files.
• Both the directory structure and the files reside
  on disk.

Directory
Files
F 1
F 2
F 3
F 4
F n
9
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)

10
Operations Performed on Directory

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

11
Actual File Size

• In Windows, disk block is 4KB
• In Unix, disk block is 512B
• Size of file reflects the actual size of data.
  
• Size on disk which is multiple of data blocks.
• Ex, if you create a text file with xyz with
  notepad, size of file will show 3, while Size on
  disk will show 4KB.

12
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 Pascal programs, all
  games, )
• Directory structure
• Single-level A single directory of all users
• Two-level Separate directory for each user
• Tree-structured Most common

13
Tree-Structured Directories

• Efficient searching
• Grouping Capability
• Absolute or relative path name
• Current directory (working directory)
• cd /spell/mail/prog
• type list
• Deleting mail ? deleting the entire subtree
  rooted by mail.

14
Shared Files/Directories

• Symbolic or soft link
• A file/directory is a pointer (contains the
  pathname) to another file/directory,
• e.g., ln s /etc/classes/ics431/class softclass
• A new i-node is created for softclass
• Takes longer to lookup as the path is traversed
• The file pointed to can be deleted or changed
• Backup might do multiple copies
• Good for linking files on other machines
• Hard link
• The file/directory points to the same
  file/directory-structure or I-node of the other
  file/directory.
• e.g., ln /etc/classes/ics431/class hardclass
• No i-node is created. Only entry in the i-node
  of directory which hardclass is under
• Faster for lookup
• Has an owner issue
• Need to keep a count in the structure for file
  deletion

15
Protection

• File owner/creator should be able to control
• what can be done
• by whom
• Types of access
• Read
• Write
• Execute
• Append
• Delete
• List

16
Access Lists and Groups

• 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
• chgrp G game

17
File-System Implementation

• Contiguous Allocation
• Each file occupies a set of contiguous blocks on
  the disk.
• Simple only starting location (block ) and
  length (number of blocks) are required.
• Random access.
• Wasteful of space (dynamic allocation problem).
• Files cannot grow.
• Linked Allocation
• Indexed Allocation

18
Linked Allocation

• Each file is a linked list of disk blocks blocks
  may be scattered anywhere on the disk.
• Simple need only starting address
• Free-space management system no waste of space
  except for pointers
• No random access
• File-allocation table (FAT) disk-space
  allocation used by MS-DOS and OS/2.
• A table used as a linked list keeping track of
  all available and used blocks. See Fig. 12-6.

19
Indexed Allocation

• Brings all pointers together into the index
  block. In Unix called (i-node).
• Random access
• Dynamic allocation without external
  fragmentation, but have overhead of index block.
• Wasted space for index block. Worse than linked
  allocation.
• How big should the index block be?

20
Use of Multi-level Index Unix i-node
21

• Where is the path stored in the inode of a
  softlink file?
• Where is list of file names in a directory inode
  is stored?

22
Directory Implementation

• Linear list of file names with pointer to the
  data blocks.
• simple to program
• time-consuming to execute
• Hash Table linear list with hash data
  structure.
• decreases directory search time
• collisions situations where two file names hash
  to the same location
• fixed size

23
Efficiency and Performance

• Efficiency dependent on
• disk allocation and directory algorithms
• types of data kept in files directory entry
• Performance
• disk cache separate section of main memory for
  frequently used blocks
• free-behind and read-ahead techniques to
  optimize sequential access
• improve PC performance by dedicating section of
  memory as virtual disk, or RAM disk.

24
Caching and Recovery
Various Disk-Caching Locations

• Consistency checker compares data in directory
  structure with data blocks on disk, and tries to
  fix inconsistencies.
• Use system programs to back up data from disk to
  another storage device (USB disks, tapes, or
  Cloud Computing dropbox).
• Recover lost file or disk by restoring data from
  backup.
• NTFS Features Logging or journaling, Encryption,
  Compression, Larger sizes than FAT, but more
  overhead (space and time).