1. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File System Implementation

2. 11.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition What to Learn? File-System Structure File-System Implementation Directory Implementation Allocation Methods of Disk Space Free-Space Management Contiguous or block-oriented Recovery from failure Remote file access: NFS

3. 11.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Layered File System

4. 11.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File-System Implementation Boot control block contains info needed by system to boot OS from that volume Volume control block contains volume details Directory structure organizes the files Per-file File Control Block (FCB) contains many details about the file

5. 11.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical File Control Block

6. 11.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition In-Memory File System Structures

7. 11.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Virtual File Systems Virtual File Systems (VFS) provide an object- oriented way of implementing file systems. VFS allows the same system call interface (the API) to be used for different types of file systems. The API is to the VFS interface, rather than any specific type of file system.

8. 11.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Schematic View of Virtual File System

9. 11.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Implementation Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute Hash Table – linear list with hash data structure. decreases directory search time collisions – situations where two file names hash to the same location

10. 11.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Allocation of Disk Blocks An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

11. 11.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation of Disk Space

12. 11.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contiguous Allocation Each file occupies a set of contiguous blocks on the disk Advantages: Simple – only starting location (block #) and length (number of blocks) are required Fast Random access Disadvantages: Not easy to grow files. Waste in space (e.g. external fragmentation)

13. 11.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block =

14. 11.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition MS-DOS: File-Allocation Table

15. 11.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Linked Allocation (Cont.) Advantages Simple – need only starting address Free-space management system – no waste of space Disadvantages: No random access Logical address mapping Block to be accessed is the Qth block in the linked chain of blocks representing the file. Displacement into block = R + 1 LA/511 Q-th block R - offset

16. 11.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition One-level Indexed Allocation Brings all pointers together into the index block Logical view index table

17. 11.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example of One-level Indexed Allocation

18. 11.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition One-level Indexed Allocation (Cont.) Advantages Support random access No external fragmentation. Disadvantages: Space overhead: need 1 block for index table Maximum file size? Assume each block is 4KB index block holds 1024 entries (4B/entry) 1024x block size = 4MB

19. 11.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two-level Indexed Allocation  outer-index Indirect pointers index table: Direct pointers File data Maximum size ? 4GB

20. 11.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Combined Scheme: UNIX UFS (4K bytes per block)

21. 11.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management Bit vector (n blocks) … 012n-1 bit[i] =  0  block[i] free 1  block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

22. 11.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Free-Space Management (Cont.) Bit map requires extra space Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Linked list (free list) Advantages:  Do not need contiguous space  No waste of space

23. 11.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Performance Optimization Disk cache – separate section of main memory for frequently used blocks Read-ahead (prefetching)– techniques to optimize sequential access improve PC performance by dedicating section of memory as virtual disk, or RAM disk

24. 11.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Disk Buffer Cache caches frequently used disk blocks Speedup file system I/O Memory-mapped I/O can also use

25. 11.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Recovery Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies Use system programs to back up data from disk to another storage device Recover lost file or disk by restoring data from backup

26. 11.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Log Structured File Systems Log structured file systems record each update to the file system as a transaction All transactions are written to a log A transaction is considered committed once it is written to the log However, the file system may not yet be updated The transactions in the log are asynchronously written to the file system When the file system is modified, the transaction is removed from the log If the file system crashes, all remaining transactions in the log must still be performed

27. 11.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Sun Network File System (NFS) Transparent shared file access among independent machines and file systems A remote directory is mounted over a local file system directory  The mounted directory looks like an integral subtree of the local file system

28. 11.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Schematic View of NFS Architecture

29. 11.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition NFS Protocol Provides a set of remote procedure calls (RPC) for remote file operations: searching for a file within a directory reading a set of directory entries manipulating links and directories accessing file attributes reading and writing files NFS servers are stateless; each request has to provide a full set of arguments Modified data must be committed to the server’s disk before results are returned to the client Support data exchange format conversion using an External Data Representation (XDR) protocol