Diff for "DiskStructures"

Back to ComputerTerms FileSystem

• Root File System
• Mounted File Systems
• Boot Block or Sector

• Super Block

  • Static parameters of the file system: FS size, Block and fragment sizes parameters affecting allocation policies
  • Problems with allocating Free blocks using the free stack.
• 4.2BSD changes to improve efficiency and robustness.

  • Cylinder groups

  • Cylinder header information: (superblock, cylinder block, and the inodes)
  • Allocation issues in Cylinder Groups
  • Global and Local allocation routines.

Disk Structures

We always have a root file system / and mounted files systems have a bit set in the inode indicating that they are mounted file systems. The inode has a device number which is looked up in the mount table.

Every file system is a seperate system resource. The first logical sector contains a boot block (sector) containing a bootstrap program.

The Superblock contains static parameters of the file system including:

• Size of hte file system
• The block and fragment sizes of data
• The free blocks list (like a stack, but maintained as a linked list orginally)

Free blocks are removed and pushed from the front of the free blocks list. This allocation operation causes a files to become more and more fragmented as disk activity continues. As a result BSD 7 used only 3% of the bandwidth available to the disk drive on average.

Changes in 4.2 BSD File allocation routines

To improve efficiency and robustness, cylinder groups where born. Cylinder groups are 1 or more consecutive cylinders grouped together for the purpose of localizing allocation. Each cylinder group contains:

• Superblock identical to every other cylinder group Superblock
• Cylinder block which contains
  • Bit map of free datablocks and fragments
  • Bit map of free inodes
  • Statistics on recent progress of allocation strategies???
• Inodes
• Data Blocks

For Example

Disk allocation happens as often as possible within a cylinder group to reduce the seek time. There are two policies that are implemented: 1) Global routine to select a desired disk block and the second is a local policy routine using the specific infomration recorded in the cylinder blocks to choose a block near the requested one. Increases in efficiency of the new Fast File System up'd bandwidth usage to 30% from 3%.

Back to ComputerTerms FileSystem