1. CS-3013 & CS-502, Summer 2006 File Systems1 Review – I/O subsystems OS responsibilities to device drivers Device API, buffering, naming, interrupt handling Installing device drivers Static vs. dynamic Discovering devices that are present Polling and identification Bitmap display device Bitblt to “paint” characters on screen Window management

2. CS-3013 & CS-502, Summer 2006 File Systems2 Network I/O & Project #3 Socket abstraction An I/O stream to a network connection Connection: A serial conversation between processes on separate machines Separate from all other connections Project #3: simple web server Listen on a port; open a socket; make a connection

3. CS-3013 & CS-502, Summer 2006 File Systems3 Review – Disks Implementer of File abstraction Storage of large amounts of data for very long times Persistence, reliability Controlled like I/O devices, but integral part of information storage subsystem Rapidly increasing capacities, dropping prices $0.5–$6.0 per gigabyte Slowly improving transfer rates, seek performance Only a factor of 5-10 in three decades!

4. CS-3013 & CS-502, Summer 2006 File Systems4 Review – Disks (continued) Organized into cylinders, tracks, sectors Random access Any sector can be read & written independently of any other Very high bandwidth for consecutive reads or writes Seek time is (often) dominating factor in performance Bad blocks are a fact of life Most detected during formatting Some occur during operation Controller or OS must step around them Seek optimization algorithms Popular study topics, less popular in real systems Long seek queues  system is out of balance

5. CS-3013 & CS-502, Summer 2006 File Systems5 Files and File Systems CS-3013 & CS-502 Operating Systems

6. CS-3013 & CS-502, Summer 2006 File Systems6 File (an abstraction) A (potentially) large amount of information or data that lives a (potentially) very long time Often much larger than the memory of the computer Often much longer than any computation Sometimes longer than life of machine itself (Usually) organized as a linear array of bytes or blocks Internal structure is imposed by application (Occasionally) blocks may be variable length (Often) requiring concurrent access by multiple processes Even by processes on different machines!

7. CS-3013 & CS-502, Summer 2006 File Systems7 File Systems and Disks User view –File is a named, persistent collection of data OS & file system view –File is collection of disk blocks –File System maps file names and offsets to disk blocks Outline for this section –Files –Directories –Implementation of Files –Implementation of Directories

8. CS-3013 & CS-502, Summer 2006 File Systems8 Fundamental ambiguity Is the file the “container of the information” or the “information” itself? Almost all systems confuse the two. Almost all people confuse the two.

9. CS-3013 & CS-502, Summer 2006 File Systems9 Example – Suppose that you e-mail me a document Later, how do either of us know that we are using the same version of the document? Windows/Outlook/Exchange/MacOS: Time-stamp is a pretty good indication that they are Time-stamps preserved on copy, drag and drop, transmission via e-mail, etc. Unix/Linux By default, time-stamps not preserved on copy, ftp, e-mail, etc. Time-stamp associated with container, not with information

10. CS-3013 & CS-502, Summer 2006 File Systems10 Rule of Thumb Almost always, application thinks in terms of the information Most systems think in terms of containers Professional Guidance: Be aware of the distinction, even when the system is not

11. CS-3013 & CS-502, Summer 2006 File Systems11 Attributes of Files Name: –Although the name is not always what you think it is! Type: –May be encoded in the name (e.g.,.cpp,.txt) Dates: –Creation, updated, last accessed, etc. –(Usually) associated with container –Better if associated with content Size: –Length in number of bytes; occasionally rounded up Protection: –Owner, group, etc. –Authority to read, update, extend, etc. Locks: –For managing concurrent access …

12. CS-3013 & CS-502, Summer 2006 File Systems12 File Metadata Information about a file –Maintained by the file system –Separate from file itself –Possibly attached to the file E.g., in block #-1 –Used by OS in variety of ways –Occasionally used by application

13. CS-3013 & CS-502, Summer 2006 File Systems13 File Types

14. CS-3013 & CS-502, Summer 2006 File Systems14 Non-attribute of files – Location Example 1: mv ~lauer/project1.doc ~cs502/public_html/S06 Example 2: –System moves file from disk block 10,000 to disk block 20,000 –System restores a file from backup May or may not be reflected in metadata

15. CS-3013 & CS-502, Summer 2006 File Systems15 Attribute anomaly – Unix File Naming ln ~lauer/project1.doc ~cs502/public_html/S06 ln ~lauer/project1.doc NewProject1.doc Unix hard links allow one file to have more than one name and/or location –The real name of a Unix file is its i-node – an internal name known only to the OS; points to metadata Hard links are not used very often in modern Unix practice –Usually safe to regard last element of path as name

16. CS-3013 & CS-502, Summer 2006 File Systems16 Operations on Files Open, Close Gain or relinquish access to a file OS returns a file handle – an internal data structure letting it cache internal information needed for efficient file access Read, Write, Truncate Read: return a sequence of n bytes from file Write: replace n bytes in file, and/or append to end Truncate: throw away all but the first n bytes of file Seek, Tell Seek: reposition file pointer for subsequent reads and writes Tell: get current file pointer Create, Delete: Conjure up a new file; or blow away an existing one

17. CS-3013 & CS-502, Summer 2006 File Systems17 File – a very powerful abstraction Documents, code Databases Very large, possibly spanning multiple disks Streams Input, output, keyboard, display Pipes, network connections, … Virtual memory backing store …

18. CS-3013 & CS-502, Summer 2006 File Systems18 File Access Methods Sequential access –Read all bytes/records from the beginning –Cannot jump around, could possibly rewind or back up –Convenient when medium was magnetic tape or punched cards Random access –Bytes/records read in any order –Essential for data base systems –Read can be … move file pointer (seek), then read or … read and then move file marker Keyed or indexed access – access items in file based on the contents of an (part of an) item in the file –Provided in older commercial operating systems (IBM ISAM) –(Usually) handled separately by database applications in modern systems

19. CS-3013 & CS-502, Summer 2006 File Systems19 Questions?

20. CS-3013 & CS-502, Summer 2006 File Systems20 Directory – A Special Kind of File A tool for users & applications to organize and find files User-friendly names Names that are meaningful over long periods of time The data structure for OS to locate files (i.e., containers) on disk

21. CS-3013 & CS-502, Summer 2006 File Systems21 Directory structures Single level –One directory per system, one entry pointing to each file –Small, single-user or single-use systems PDA, cell phone, etc. Two-level –Single “master” directory per system –Each entry points to one single-level directory per user –Uncommon in modern operating systems Hierarchical –Any directory entry may point to Individual file Another directory –Used in most modern operating systems

22. CS-3013 & CS-502, Summer 2006 File Systems22 Directory Considerations Efficiency – locating a file quickly. Naming – convenient to users. Separate users can use same name for separate files. The same file can have different names for different users. Names need only be unique within a directory Grouping – logical grouping of files by properties e.g., all Java programs, all games, …

23. CS-3013 & CS-502, Summer 2006 File Systems23 Directory Organization – Hierarchical Most systems support idea of current (working) directory –Absolute names – fully qualified from root of file system /usr/group/foo.c –Relative names – specified with respect to working directory foo.c –A special name – the working directory itself “.” Modified Hierarchical – Acyclic Graph (no loops) and General Graph –Allow directories and files to have multiple names –Links are file names (directory entries) that point to existing (source) files

24. CS-3013 & CS-502, Summer 2006 File Systems24 Links Hard links: bi-directional relationship between file names and file –A hard link is directory entry that points to a source file’s metadata –Metadata maintains reference count of the number of hard links pointing to it – link reference count –Link reference count is decremented when a hard link is deleted –File data is deleted and space freed when the link reference count goes to zero Symbolic (soft) links: uni-directional relationship between a file name and the file –Directory entry points to new metadata –Metadata points to the source file –If the source file is deleted, the link pointer is invalid

25. CS-3013 & CS-502, Summer 2006 File Systems25 Path Name Translation Assume that I want to open “/home/lauer/foo.c” fd = open(“/home/lauer/foo.c”, O_RDWR); File System does the following –Opens directory “/” – the root directory is in a known place on disk –Search root directory for the directory home and get its location –Open home and search for the directory lauer and get its location –Open lauer and search for the file foo.c and get its location –Open the file foo.c –Note that the process needs the appropriate permissions File Systems spend a lot of time walking down directory paths –This is why open calls are separate from other file operations –File System attempts to cache prefix lookups to speed up common searches

26. CS-3013 & CS-502, Summer 2006 File Systems26 Directory Operations Create: Make a new directory Add, Delete entry: Invoked by file create & destroy, directory create & destroy Find, List: Search or enumerate directory entries Rename: Change name of an entry without changing anything else about it Link, Unlink: Add or remove entry pointing to another entry elsewhere Introduces possibility of loops in directory graph Destroy: Removes directory; must be empty

27. CS-3013 & CS-502, Summer 2006 File Systems27 More on Directories Fundamental mechanism for interpreting file names in an operating system Orphan: a file not named in any directory Cannot be opened by any application (or even OS) (Often) does not even have name! Tools FSCK – check & repair file system, find orphans Delete_on_close attribute Special directory entry: “..”  parent in hierarchy Essential for maintaining integrity of directory system Useful for relative naming

28. CS-3013 & CS-502, Summer 2006 File Systems28 Break

29. CS-3013 & CS-502, Summer 2006 File Systems29 Implementation of Files Map file abstraction to physical disk blocks Some goals –Efficient in time, space, use of disk resources –Fast enough for application requirements –Effective for a wide variety of file sizes Many small files (< 1 page) Huge files (100’s of gigabytes, terabytes, spanning disks) Everything in between

30. CS-3013 & CS-502, Summer 2006 File Systems30 File Allocation Schemes Contiguous –Blocks of file stored in consecutive disk sectors –Directory points to first entry Linked –Blocks of file scattered across disk, as linked list –Directory points to first entry Indexed –Separate index block contains pointers to file blocks –Directory points to index block

31. CS-3013 & CS-502, Summer 2006 File Systems31 Contiguous Allocation Ideal for large, static files –Databases, fixed system structures, OS code –CD-ROM, DVD Simple address calculation –Block address  sector address Fast multi-block reads and writes –Minimize seeks between blocks Prone to fragmentation when … Files come and go Files change size –Similar to unpaged virtual memory

32. CS-3013 & CS-502, Summer 2006 File Systems32 Bad Block Management – Contiguous Allocation Bad blocks must be concealed Foul up the block-to-sector calculation Methods Spare sectors in each track, remapped by formatting Look-aside list of bad sectors –Check each sector request against hash table –If present, substitute a replacement sector behind the scene Handling Disk controller, invisible to OS Lower levels of OS file system; invisible to application

33. CS-3013 & CS-502, Summer 2006 File Systems33 Contiguous Allocation – Extents Extent: a contiguously allocated subset of a file Directory entry contains –(For file with one extent) the extent itself –(For file with multiple extents) pointer to an extent block describing multiple extents Advantages –Speed, ease of address calculation of contiguous file –Avoids (some of) the fragmentation issues –Can be extended to support files across multiple disks Disadvantages –Degenerates to indexed allocation in Unix-like systems

34. CS-3013 & CS-502, Summer 2006 File Systems34 Linked Allocation Blocks scattered across disk –Each block contains pointer to next block –Directory points to first and last blocks –Sector header: Pointer to next block ID and block number of file

35. CS-3013 & CS-502, Summer 2006 File Systems35 Linked Allocation Advantages –No space fragmentation! –Easy to create, extend files –Ideal for lots of small files Disadvantages –Lots of disk arm movement –Space taken up by links –Sequential access only!

36. CS-3013 & CS-502, Summer 2006 File Systems36 Variation on Linked Allocation – File Allocation Table Instead of a link on each block, put all links in one table –the FAT (File Allocation Table) One entry per physical block in disk –Directory points to first & last blocks of file –Each block points to next block (or EOF)

37. CS-3013 & CS-502, Summer 2006 File Systems37 FAT File Systems Advantages –Advantages of Linked File System –FAT can be cached in memory –Searchable at CPU speeds, pseudo-random access Disadvantages –Limited size, not suitable for very large disks –FAT cache describes entire disk, not just open files! –Not fast enough for large databases Used in MS-DOS, early Windows systems

38. CS-3013 & CS-502, Summer 2006 File Systems38 Disk Defragmentation Re-organize blocks in disk so that file is (mostly) contiguous Link or FAT organization preserved Minimizes disk arm movement during sequential accesses

39. CS-3013 & CS-502, Summer 2006 File Systems39 Bad Block Management – Linked and FAT Systems In OS:– format all sectors of disk Don’t reserve any spare sectors Allocate bad blocks to a hidden file for the purpose If a block becomes bad, append to the hidden file Advantages Very simple No look-aside or sector remapping needed Totally transparent without any hidden mechanism

40. CS-3013 & CS-502, Summer 2006 File Systems40 Indexed Allocation i-node: –Part of file metadata –Data structure lists the sector address of each block of file Advantages –True random access –Only i-nodes of open files need to be cached –Supports small and large files

41. CS-3013 & CS-502, Summer 2006 File Systems41 Unix i-nodes Direct blocks: –Pointers to first n sectors Single indirect table: –Extra block containing pointers to blocks n+1.. n+m Double indirect table: –Extra block containing single indirect blocks …

42. CS-3013 & CS-502, Summer 2006 File Systems42 Indexed Allocation Access to every block of file is via i-node Bad block management –Same as Linked/FAT systems Disadvantage –Not as fast as contiguous allocation for large databases Requires reference to i-node for every access vs. Simple calculation of block to sector address

43. CS-3013 & CS-502, Summer 2006 File Systems43 Questions?

44. CS-3013 & CS-502, Summer 2006 File Systems44 Implementation of Directories A list of [name, information] pairs Must be scalable from very few entries to very many Name: User-friendly, variable length Any language Fast access by name Information: File metadata (itself) Pointer to file metadata block (or i-node) on disk Pointer to first & last blocks of file Pointer to extent block(s) …

45. CS-3013 & CS-502, Summer 2006 File Systems45 Very Simple Directory Short, fixed length names Attribute & disk addresses contained in directory MS-DOS, etc. name1attributesname2attributesname3attributesname4attributes …

46. CS-3013 & CS-502, Summer 2006 File Systems46 Simple Directory Short, fixed length names Attributes in separate blocks (e.g., i-nodes) Attribute pointers are disk addresses (or i-node numbers) Older Unix versions name1name2name3name4… i-node Data structures containing attributes

47. CS-3013 & CS-502, Summer 2006 File Systems47 More Interesting Directory Variable length file names –Stored in heap at end Modern Unix, Windows Linear or logarithmic search for name Compaction needed after –Deletion, Rename attributes … name1 longer_na me3 very_long_n ame4 name2 …

48. CS-3013 & CS-502, Summer 2006 File Systems48 Very Large Directories Hash-table implementation Each hash chain like a small directory with variable-length names Must be sorted for listing

49. CS-3013 & CS-502, Summer 2006 File Systems49 Free Block Management Bitmap –Very compact on disk –Expensive to search –Supports contiguous allocation Free list –Linked list of free blocks Each block contains pointer to next free block –Only head of list needs to be cached in memory –Very fast to search and allocate –Contiguous allocation vary difficult

50. CS-3013 & CS-502, Summer 2006 File Systems50 Free Block Management Bit Vector … 012n-1 bit[i] =  0  block[i] free 1  block[i] occupied Free block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

51. CS-3013 & CS-502, Summer 2006 File Systems51 Free Block Management Bit Vector (continued) Bit map –Must be kept both in memory and 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 –Similarly for set of contiguous blocks Potential for lost blocks in event of crash! –Discussion:– How do we solve this problem?

52. CS-3013 & CS-502, Summer 2006 File Systems52 Free Block Management Linked List Linked list of free blocks Not in order! Cache first few free blocks in memory Head of list must be stored both On disk & in memory Each block written to disk when freed Potential for losing blocks?

53. CS-3013 & CS-502, Summer 2006 File Systems53 Bread Next Topic

54. CS-3013 & CS-502, Summer 2006 File Systems54 Raw Disk Layout Track format – n sectors –200 < n < 2000 in modern disks –Some disks have fewer sectors on inner tracks Inter-sector gap –Enables each sector to be read or written independently Sector format –Sector address: Cylinder, Track, Sector –Optional header –Data –Each field separated by small gap and with its own CRC Sector length –Almost all operating systems specify uniform sector length –512 – 4096 bytes