File system Implementation

Overview

• In this chapter, we study very simple file system (vsfs)
  • Basic on-disk structures, access methods, and various policies of vsfs
• We will study...
  • How can we build a simple file system?
  • What structures are needed on the disk?
  • What do they need to track?
  • How are they accessed?
• What types of data structures are utilized by the file system?
• How file system organize its data and metadata?
• Understand access methods of a file system.
  • open(), read(), write(), etc.

Overall Organization

• Let’s develop the overall organization of the file system data structure.
• Divide the disk into blocks.
  • Block size is 4 KB.
  • The blocks are addressed from 0 to N -1.

Data region in file system

• Reserve data region to store user data

• File system has to track which data block comprise a file, the size of
  the file, its owner, etc.

Inode table in file system

• Reserve some space for inode table
  • This holds an array of on-disk inodes.
  • Ex) inode tables : 3 ~ 7, inode size : 256 bytes
    • 4-KB block can hold 16 inodes.
    • The file system contains 80 inodes. (maximum number of files)

Allocation structures

• This is to track whether inodes or data blocks are free or allocated.
• Use bitmap, each bit indicates free(0) or in-use(1)
  • data bitmap (d) : for data region
  • inode bitmap (i) : for inode table