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Advanced Operating Systems

File System Internals

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UNIX File System Organisation

• The Traditional UNIX organisation from Ed 7 onwards has been

– Boot Block, Super Block, Inodes, Data

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UNIX File System Organisation

• Searching and allocation is done through logical device and inumber

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UNIX File System Organisation

• Problems of Traditional UNIX FS– Fragmentation of disc– No way to use disc geometry– Short file names

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Berkeley Fast File System

• Restructured File System organisation using cylinder groups– 1 or more cylinders per group

– Identical superblocks in cylinder group

– Superblocks offset for each in cylinder group

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Berkeley Fast File System

• Inode/file allocation– File inode is allocated in parent directory

cylinder group– New directory placed in new cylinder group– Large and small files are segregated– Use of 8k byte blocks & 1k fragements

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Berkeley Fast File System

• Filename size changes– 255 character file names

• Changed directory organisation

– Quota system introduced– Symbolic links introduced– File truncation introduced

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Linux file system

• Added VFS – Virtual File System– Allows other system to be added– Clean, object orientated, design of system

• 3 parts– Inode object– File object– File system object

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Virtual File System

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Virtual File System

• VFS Objects define an action for each file system

• Each object has a pointer into a table of functions for each file system implementation

• File system object is the hierarchy

• Files found through fs-inode paring