CS-241 Magnetic Disks

• The Disk:

  • Physical structure
  • accessing data
  • Accessing data is much faster if data is contiguous on disk.

• A File: A file is a sequence of blocks on disk.

  • A block is often either 512 or 1024 bytes.
  • Entire file can be stored consecutively on disk - problematic when files are deleted and added.
  • Normally, stored as a linked list on disk. Potential problem with random access but can be salvaged using file index.

• Operating system

  • OS needs some way of locating file on disk. Uses a file directory of some sort.
    • directory entry for unix: i-node, file name
    • entry for ms-dos: file name, ext, ... first block number into FAT, size

  • File Storage

    • Unix: i-node - list of block addresses along with other info.
    • FAT: File Allocation Table (ms-dos, and windows 95/98)
    • NTFS: New Technology File System - Windows NT, 2000, XP

• FAT

  • Each disk has a table with on entry for each disk block.
  • The directory entry for each file gives the block number for the first block.
  • Each slot in the FAT gives the block number of the next block.
  • Example

• The Free List

  • To allocate space for a file, need to know what blocks are free. This is called the free list.
  • Use linked list.
  • Example

• Disk fragmentations

  • Block of a file are not contiguous.
  • Fragmentation slows down access.
  • Can defragment disk.