CS537 11/18

Today

• FS API
• FS Implementation

FS API

• File

  • byte array
  • has low-level name (number)

• Directory

  • Map human-readable names -> low-level names

• e.g. file “main.c”

  • low-level #: 1000
  • directory: “main.c” -> 1000

• Access

  • open, read, close

• Grow

  • open, write, close

• Write to arbitrary point in the file:

  • lseek

• Delete

  • unlink

• FS: tracks info about each file

  • “Meta data”
  • What info is there per file?

Directory

• Just a special type of file
• metadata: inode
• data: contents of directory
  • Map:
  • “main.c” -> inode #1000
  • “main” -> inode #1001
  • … etc.
  • all directories also store:
  • “.” (dot) -> inode # of current directory
  • “..” (dot dot) -> inode # of parent directory

File System Hierarchy

           /   (root directory)
     /     |    \
   dir  file.c  dir2
                  |     \
                dir3   file2.c
copy

• Absolute pathname:
  • starts at root, and includes the entire path to file or dir
• Relative pathname:
  • current working directory (CWD)
    • cd => move shell to “location” in hierarchy
    • cd /dir2 CWD: dir2
      • “file2.c”
      • “./file2.c”
      • “../file2.c”
      • “../dir2/file2.c”
• Path traversal:
  • open("/a/b/c/d/main.c", O_RDONLY)
  • read root dir /
    • look for “a” => a’s inode #
  • read “a”
    • look for “b” => b’s inode #
  • …
  • read “d”
    • look for “man.c” => main.c’s inode #
• Link (Hard Link)
  • program: ln

  • directory    inode #
    "foo"        8388622
    "foo2"       8388622
    "foo3"       8388622
    copy

  • rm: unlink inode from file name
  • “last” unlink: removes contents from disk, etc.

File System Implementation

• On-disk structures static
• Access methods operation
• Disk: [blocks] => 4KB
• Inode Bitmap
  • tracks which inodes free/not
• Data Bitmap
  • track which block free/not
• Super Block:
  • metadata: information about entire file system
  • e.g. => size, where data region is, etc.
  • magic #, type