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
Data Blocks |
Superblock |
Cylinder Block |
Inodes |
Data Blocks |
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