File System Implementation

1. File System Implementation CSCI 315 Operating Systems Design

2. 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. CSCI 315 Operating Systems Design

3. 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. CSCI 315 Operating Systems Design

4. General Graph Directory CSCI 315 Operating Systems Design

5. 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. CSCI 315 Operating Systems Design

6. 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. unmounted existing CSCI 315 Operating Systems Design

7. 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. CSCI 315 Operating Systems Design

8. 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) CSCI 315 Operating Systems Design

9. 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. CSCI 315 Operating Systems Design

10. 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. owner group public chmod 761 game Associate a group with a file: chgrp G game CSCI 315 Operating Systems Design

11. 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. CSCI 315 Operating Systems Design

12. Layered File System CSCI 315 Operating Systems Design

13. A Typical File Control Block CSCI 315 Operating Systems Design

14. In-Memory File System Structures file open file read CSCI 315 Operating Systems Design

15. 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. CSCI 315 Operating Systems Design

16. Schematic View of Virtual File System same API for all file system types ext3 FAT 32 NFS CSCI 315 Operating Systems Design

17. Directory Implementation The directory is a symbol table that maps file names to pointers that lead to the blocks comprising a file. • 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. CSCI 315 Operating Systems Design

18. Allocation Methods An allocation method refers to how disk blocks are allocated for files: • Contiguous allocation, • Linked allocation, • Indexed allocation. CSCI 315 Operating Systems Design

19. 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. CSCI 315 Operating Systems Design

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

21. Contiguous Allocation of Disk Space CSCI 315 Operating Systems Design

22. 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. CSCI 315 Operating Systems Design

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

24. Linked Allocation (Cont.) • Simple – need only starting address • Free-space management system – no waste of space • No random access • Mapping Q LA/511 R 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. CSCI 315 Operating Systems Design

25. Linked Allocation CSCI 315 Operating Systems Design

26. File-Allocation Table CSCI 315 Operating Systems Design

27. Indexed Allocation • Brings all pointers together into the index block. • Logical view. index table CSCI 315 Operating Systems Design

28. Example of Indexed Allocation CSCI 315 Operating Systems Design

29. 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. Q LA/512 R Q = displacement into index table R = displacement into block CSCI 315 Operating Systems Design

30. 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). Q1 LA / (512 x 511) R1 Q1= block of index table R1is used as follows: Q2 R1 / 512 R2 Q2 = displacement into block of index table R2 displacement into block of file: CSCI 315 Operating Systems Design

31. Indexed Allocation – Mapping (Cont.) • Two-level index (maximum file size is 5123) Q1 LA / (512 x 512) R1 Q1 = displacement into outer-index R1 is used as follows: Q2 R1 / 512 R2 Q2 = displacement into block of index table R2 displacement into block of file: CSCI 315 Operating Systems Design

32. Indexed Allocation – Mapping (Cont.)  outer-index file index table CSCI 315 Operating Systems Design

33. Combined Scheme: UNIX (4K bytes per block) CSCI 315 Operating Systems Design

34. Free-Space Management • Bit vector (n blocks) 0 1 2 n-1 … 0  block[i] free 1  block[i] occupied bit[i] =  Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit CSCI 315 Operating Systems Design

35. Free-Space Management (Cont.) • Bit map requires extra space. Example: block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) • Easy to get contiguous files • Linked list (free list) • Cannot get contiguous space easily • No waste of space • Grouping • Counting CSCI 315 Operating Systems Design

36. 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 CSCI 315 Operating Systems Design

37. Linked Free Space List on Disk CSCI 315 Operating Systems Design

38. 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. CSCI 315 Operating Systems Design

39. Various Disk-Caching Locations CSCI 315 Operating Systems Design

40. 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. CSCI 315 Operating Systems Design

41. I/O Without a Unified Buffer Cache CSCI 315 Operating Systems Design

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

43. I/O Using a Unified Buffer Cache CSCI 315 Operating Systems Design

44. 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. CSCI 315 Operating Systems Design

45. 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. CSCI 315 Operating Systems Design