1. 04/05/2004CSCI 315 Operating Systems Design1 File System Implementation

2. 04/05/2004CSCI 315 Operating Systems Design2 Acyclic-Graph Directories Have shared subdirectories and files. links: soft (symbolic) hard Unix: ln (read man page); need to keep a reference count on each file or directory.

3. 04/05/2004CSCI 315 Operating Systems Design3 Acyclic-Graph Directories (Cont.) Different names (aliasing) for the same file or directory. If dict deletes list  dangling pointer. Solutions: –Backpointers, so we can delete all pointers. Variable size records a problem. –Backpointers using a daisy chain organization. –Entry-hold-count solution.

4. 04/05/2004CSCI 315 Operating Systems Design4 General Graph Directory

5. 04/05/2004CSCI 315 Operating Systems Design5 General Graph Directory (Cont.) How do we guarantee no cycles? –Allow only links to file not subdirectories. –Garbage collection. –Every time a new link is added use a cycle detection algorithm to determine whether it is OK.

6. 04/05/2004CSCI 315 Operating Systems Design6 File System Mounting A file system (partition) must be mounted before it can be accessed. Mounting allows one to attach the file system on one device to the file system on another device. A unmounted file system needs to be attached to a mount point before it can be accessed. existing unmounted

7. 04/05/2004CSCI 315 Operating Systems Design7 File Sharing Sharing of files on multi-user systems is desirable. Sharing may be done through a protection scheme. On distributed systems, files may be shared across a network. Network File System (NFS) is a common distributed file- sharing method.

8. 04/05/2004CSCI 315 Operating Systems Design8 Protection File owner/creator should be able to control: –what can be done, –by whom. Types of access: –Read, –Write, –Execute, –Append, –Delete, –List. Discretionary Access Control (DAC)

9. 04/05/2004CSCI 315 Operating Systems Design9 Protection Mandatory Access Control (MAC): –System policy: files tied to access levels = (public, restricted, confidential, classified, top-secret). –Process also has access level: can read from and write to all files at same level, can only read from files below, can only write to files above. Role-Based Access Control (RBAC): –System policy: defines “roles” (generalization of the Unix idea of groups). –Roles are associated with access rules to sets of files and devices. –A process can change roles (in a pre-defined set of possibilities) during execution.

10. 04/05/2004CSCI 315 Operating Systems Design10 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7  1 1 1 RWX b) group access 6  1 1 0 RWX c) public access 1  0 0 1 Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. ownergrouppublic chmod761game Associate a group with a file: chgrp G game

11. 04/05/2004CSCI 315 Operating Systems Design11 File-System Structure File structure: –Logical storage unit, –Collection of related information. File system resides on secondary storage (disks). File system is organized into layers. File control block – storage structure consisting of information about a file.

12. 04/05/2004CSCI 315 Operating Systems Design12 Layered File System

13. 04/05/2004CSCI 315 Operating Systems Design13 A Typical File Control Block

14. 04/05/2004CSCI 315 Operating Systems Design14 In-Memory File System Structures file open file read

15. 04/05/2004CSCI 315 Operating Systems Design15 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.

16. 04/05/2004CSCI 315 Operating Systems Design16 Schematic View of Virtual File System ext3FAT 32NFS same API for all file system types

17. 04/05/2004CSCI 315 Operating Systems Design17 Directory Implementation Linear list of file names with pointer to the data blocks: –simple to program, but… –time-consuming to execute. Hash Table: –decreases directory search time, –collisions – situations where two file names hash to the same location, –fixed size. The directory is a symbol table that maps file names to pointers that lead to the blocks comprising a file.

18. 04/05/2004CSCI 315 Operating Systems Design18 Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation, Linked allocation, Indexed allocation.

19. 04/05/2004CSCI 315 Operating Systems Design19 Contiguous Allocation Each file occupies a set of contiguous blocks on the disk. Simple – only starting location (block #) and length (number of blocks) are required. Random access. Wasteful of space (dynamic storage-allocation problem). Files cannot grow.

20. 04/05/2004CSCI 315 Operating Systems Design20 Contiguous Allocation Mapping from logical to physical. LA/512 Q R –Block to be accessed = ! + starting address –Displacement into block = R

21. 04/05/2004CSCI 315 Operating Systems Design21 Contiguous Allocation of Disk Space

22. 04/05/2004CSCI 315 Operating Systems Design22 Extent-Based Systems Many newer file systems (i.e. Veritas File System) use a modified contiguous allocation scheme. Extent-based file systems allocate disk blocks in extents. An extent is a contiguous set of block. Extents are allocated for each file. A file consists of one or more extents.

23. 04/05/2004CSCI 315 Operating Systems Design23 Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. pointer block =

24. 04/05/2004CSCI 315 Operating Systems Design24 Linked Allocation (Cont.) Simple – need only starting address Free-space management system – no waste of space No random access Mapping Block to be accessed is the Qth block in the linked chain of blocks representing the file. Displacement into block = R + 1 File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2. LA/511 Q R

25. 04/05/2004CSCI 315 Operating Systems Design25 Linked Allocation

26. 04/05/2004CSCI 315 Operating Systems Design26 File-Allocation Table

27. 04/05/2004CSCI 315 Operating Systems Design27 Indexed Allocation Brings all pointers together into the index block. Logical view. index table

28. 04/05/2004CSCI 315 Operating Systems Design28 Example of Indexed Allocation

29. 04/05/2004CSCI 315 Operating Systems Design29 Indexed Allocation (Cont.) Need index table Random access Dynamic access without external fragmentation, but have overhead of index block. Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 words. We need only 1 block for index table. LA/512 Q R Q = displacement into index table R = displacement into block

30. 04/05/2004CSCI 315 Operating Systems Design30 Indexed Allocation – Mapping (Cont.) Mapping from logical to physical in a file of unbounded length (block size of 512 words). Linked scheme – Link blocks of index table (no limit on size). LA / (512 x 511) Q1Q1 R1R1 Q 1 = block of index table R 1 is used as follows: R 1 / 512 Q2Q2 R2R2 Q 2 = displacement into block of index table R 2 displacement into block of file:

31. 04/05/2004CSCI 315 Operating Systems Design31 Indexed Allocation – Mapping (Cont.) Two-level index (maximum file size is 512 3 ) LA / (512 x 512) Q1Q1 R1R1 Q 1 = displacement into outer-index R 1 is used as follows: R 1 / 512 Q2Q2 R2R2 Q 2 = displacement into block of index table R 2 displacement into block of file:

32. 04/05/2004CSCI 315 Operating Systems Design32 Indexed Allocation – Mapping (Cont.)  outer-index index table file

33. 04/05/2004CSCI 315 Operating Systems Design33 Combined Scheme: UNIX (4K bytes per block)

34. 04/05/2004CSCI 315 Operating Systems Design34 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

35. 04/05/2004CSCI 315 Operating Systems Design35 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) Easy to get contiguous files Linked list (free list) –Cannot get contiguous space easily –No waste of space Grouping Counting

36. 04/05/2004CSCI 315 Operating Systems Design36 Free-Space Management (Cont.) Need to protect: –Pointer to free list –Bit map Must be kept on disk Copy in memory and disk may differ. Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk. –Solution: Set bit[i] = 1 in disk. Allocate block[i] Set bit[i] = 1 in memory

37. 04/05/2004CSCI 315 Operating Systems Design37 Linked Free Space List on Disk

38. 04/05/2004CSCI 315 Operating Systems Design38 Efficiency and Performance Efficiency dependent on: –disk allocation and directory algorithms –types of data kept in file’s directory entry Performance –disk cache – separate section of main memory for frequently used blocks –free-behind and read-ahead – techniques to optimize sequential access –improve PC performance by dedicating section of memory as virtual disk, or RAM disk.

39. 04/05/2004CSCI 315 Operating Systems Design39 Various Disk-Caching Locations

40. 04/05/2004CSCI 315 Operating Systems Design40 Page Cache A page cache caches pages rather than disk blocks using virtual memory techniques. Memory-mapped I/O uses a page cache. Routine I/O through the file system uses the buffer (disk) cache. This leads to the following figure.

41. 04/05/2004CSCI 315 Operating Systems Design41 I/O Without a Unified Buffer Cache

42. 04/05/2004CSCI 315 Operating Systems Design42 Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O.

43. 04/05/2004CSCI 315 Operating Systems Design43 I/O Using a Unified Buffer Cache

44. 04/05/2004CSCI 315 Operating Systems Design44 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 (floppy disk, magnetic tape). Recover lost file or disk by restoring data from backup.

45. 04/05/2004CSCI 315 Operating Systems Design45 Log Structured File Systems Log structured (or journaling) 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.