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File System Implementation

Chapter 11

File-System Structure

• A file system poses two distinct design problems - Defining how the file system should look to the user - Creating algorithms and data structures to map the logical file system onto the physical device
• File system resides on secondary storage (disks)
• File system organized into layers.
  • Each level uses the lower level to create new features for use by higher levels
• File control block – storage structure consisting of information about a file

File-System Implementation

• Several on-disk and in-memory structures are used

to implement a file system

• On-disk structures
  • A boot control block
  • A partition control block (superblock)
  • A directory structure
  • file control blocks
• In-memory structures
  • An in-memory partition table
  • An in-memory directory structure
  • The system-wide open-file table
  • The per-process open-file table

Creating a File

• To create a new file the application program calls the logical

file system (which knows the format of the directory structures)

• Allocates a new FCB
• Reads the appropriate directory into memory
• Updates the directory with new file name and FCB
• Writes back to disk
• Some operating systems (UNIX) treat a directory exactly as a

file, other operating systems (Windows), implement separate system calls for files and directories and treat directories separate from files.

In-Memory File System Structures

(a) refers to opening a file. (b) refers to reading a file.

Closing a File

• After all I/O operations are complete a file should be

closed

• The per-process table entry is removed and the system- wide entry’s open count is decremented
• When all users that have opened the file close it, the updated file information is copied back to the disk-based directory structure and the system-wide open-file table entry is removed
• Some systems Use a caching a scheme. All

information about an open file, except for its actual

data blocks, is in memory

Directory Implementation

• Linear List
  • Uses a linear list of file names with pointers to data blocks, requires a linear search to find a particular entry
  • Simple to program but time-consuming to execute
• Hash Table
  • Uses a linear list to stores directory entries but uses hashing to find the entry
  • Hashing can greatly decrease the directory search time
  • Handle collisions – situations where two file names hash to the same location
  • Major difficulties with a hash table are its fixed size and dependence on the hash function

Allocation Methods

• An allocation method refers to how disk blocks are allocated

for files

• Three major methods of allocating disk space are in wide use
  • Contiguous allocation
  • Linked allocation
  • Indexed allocation
• Each method has its advantages and disadvantages
• Some systems support all three but more commonly a system

will use one particular method

Contiguous Allocation of Disk Space

Contiguous Allocation (Cont.)

• Contiguous allocation has some problems
  • Dynamic storage-allocation
    • How to satisfy a request of size n from a list of free blocks
  • External fragmentation
    • Free space is broken into chunks and the largest chunk is insufficient for a request
  • Determining how much space is needed for a file
    • Allocate too little and the file may not be extended
    • Allocate too much and space is wasted
  • File cannot grow

Linked Allocation

• Solves all the problems of contiguous allocation
• Each file is a linked list of disk blocks: blocks may be scattered

anywhere on the disk

• The directory contains a pointer to the first and last blocks of

a file

block = pointer

Linked Allocation

File-Allocation Table

Indexed Allocation

• Solves the external-fragmentation and size-declaration

problems of contiguous allocation

• Supports direct access by bringing all the pointers together

into the index block

• Each file has its own index block, which is an array of disk-

block addresses

index table