1. Chapter 8 File Management

2. 8.1 Introduction
Data should be organized in some convenient and efficient manner. In particular, users should be able to:
Put data into files
Find and use files that have previously been created

3. File System
Set of OS Services that provides Files and Directories for user applications

4. 8.2 Files
A file is simply a sequence of bytes that have been stored in some device (storage) on the computer

5. Files
Those bytes will contain whatever data we would like to store in the file such as:
A text file just containing characters that we are interested in
A word processing document file that also contains data about how to format the text
A database file that contains data organized in multiple tables.
In general, the File Management system does not have any knowledge about how the data in a file is organized. That is the responsibility of the application programs that create and use the file.

6. Permanent (non-volatile) Storage Devices
Disk Drives
Flash Memory (Memory stick)
CDs and DVDs
Magnetic tape drives

7. 8.2.1 File Attributes Name Type Location
Symbolic (Human-readable) name of the file
Type
Executable file, print file, etc.
Location
Where file is on disk

8. File Attributes Size Protection Time, date
Who can read, write file, etc.
Time, date
When file was created, modified, accessed

9. 8.2.2 Folders An important attribute of folders is the Name
Typically, a folder may contain Files and other Folders (commonly called sub-folders or sub-directories)
This results in a Tree Structure of Folder and Files.

10. Folder/Directory Tree Structure

11. 8.2.3 Pathnames
The pathname of a file specifies the sequence of folders one must traverse to travel down the tree to the file.
This pathname actually describes the absolute path of the file, which is the sequence of folders one must travel from the root of the tree to the desired file.
A relative path describes the sequence of Folders one must traverse starting at some intermediate place on the absolute path.
The Absolute path provides a unique identification for a file. Two different files can have the same filename as long as the resulting pathnames are unique.

12. File Links
Allow a directory entry to point to a file (or entry) that is not directly below it in the tree structure
Unix: Symbolic Link
Windows: Shortcut

13. Link in Directory Tree Structure

14. 8.3 Access Methods
An access method describes the manner and mechanisms by which a process accesses the data in a file.
There are two common access methods:
Sequential
Random (or Direct)

15. File Operations
When a process needs to use a file, there are a number of operations it can perform:
Open
Close
Read
Write

16. Create File Allocate space for file
Make entry for file in the Directory

17. 8.3.1 Open File Make files accessible for read/write operations
Locates files in the Directory
Returns internal ID for the file
Commonly called a Handle
handle = open(filename, parameters)

18. File Open

19. 8.3.2 Close File Makes file no longer accessible from application
Deletes the Handle created by Open

20. File Close

21. 8.3.3 Read File System call specifies: Handle from Open call
Memory Location, length of information to be read
Possibly, location in the file where data is to be read from
read(file handle, buffer)
read(file handle, buffer, length)

22. Read File Uses Handle to locate file on disk
Uses file’s Read Pointer to determine the position in the file to read from
Update file’s Read Pointer

23. 8.3.4 Write File System call specifies: Handle from Open call
Location, length of information to be written
Possibly, location in the file where data is to be written
write(file handle,buffer,length)

24. Write File Use Handle to locate file on disk
Use file’s Write pointer to determine the position in the file to write to
Update file’s Write Pointer

25. Delete File Deletes entry for file in Directory
De-allocates disk space used by the file

26. 8.3.5 Sequential Access
If the process has opened a file for sequential access, the File Management subsystem will keep track of the current file position for reading and writing.
To carry this out, the system will maintain a file pointer that will be the position of the next read or write.

27. File Pointer
The value of the file pointer will be initialized during Open to one of two possible values
Normally, this value will be set to 0 to start the reading or writing at the beginning of the file.
If the file is being opened to append data to the file, the File Position pointer will be set to the current size of the file.
After each read or write, the File Position Pointer will be incremented by the amount of data that was read or written.

28. 8.3.6 Streams, Pipes, and I/O Redirection
A Stream is the flow of data bytes, one byte after another, into the process (for reading) and out of the process (for writing).
This concept applies to Sequential Access and was originally invented for network I/O, but several modern programming environments (e.g. Java, C#) have also incorporated it.

29. Standard I/O Standard Input Standard Output Defaults to keyboard
Defaults to console

30. I/O Redirection Standard Input can come from a file
app.exe < def.txt
Standard Output can go to a file
App.exe > def.txt
Standard Output from one application can be Standard Input for another
App1.exe | app2.exe
Called a Pipe

31. A Pipe

32. Pipe
A Pipe is a connection that is dynamically established between two processes.
When a process reads data, the data will come from another process rather than a file. Thus, a pipe has a process at one end that is writing to the pipe and another process reading data at the other end of the pipe.
It is often the situation that one process will produce output that another process needs for input.
Rather than having the first process write to a file and the second process read that file, we can save time by having each process communicate via a pipe.

33. Pipe and Performance
Using a pipe can improve system performance in two ways:
By not using a file, the applications save time by not using disk I/O.
A pipe has the characteristic that the receiving process can read whatever data has already been written. Thus we do not need to wait until the first process has written all of the data before we start executing the second process. This creates a pipeline similar to an automobile assembly line to speed up overall performance.

34. 8.4 Directory Functions Search for a file Create a file Delete a file
List a directory
Rename a file
Traverse the file system

35. 8.5 File Space Allocation Contiguous
File is allocated contiguous disk space

36. File System Implementation
A possible file system layout
A Master Boot Record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices. The MBR holds the information on how the logical partitions, containing file systems, are organized on that medium.

37. Implementing Files (1)
(a) Contiguous allocation of disk space for 7 files
(b) State of the disk after files D and E have been removed

38. Contiguous Allocation
Advantages
Simple to implement
Good disk I/O performance
Disadvantages
Need to know max file size ahead of time
Probably will waste disk space
Necessary space may not be available

39. Contiguous Allocation
Read/Write Disk Address Calculation

40. 8.5.1 Cluster Allocation Cluster Allocation
Disk space allocated in blocks
Space allocated as needed

41. Cluster Allocation

42. Linked list allocation using a file allocation table in RAM
Implementing Files (3)
Linked list allocation using a file allocation table in RAM

43. Implementing Files (4)
An example i-node

44. Cluster Allocation Advantages Disadvantages
Tends not to waste disk space
Disadvantages
Additional overhead to keep track of clusters
Can cause poor disk I/O performance
May limit maximum size of File System

45. Cluster Performance Clusters tend to be scattered around the disk
This is called External Fragmentation
Can cause poor performance as disk arm needs to move a lot
Requires De-fragmentation utility

46. Cluster Performance Large clusters can reduce External Fragmentation
If lots of small files, then space will be wasted inside each cluster
This is called Internal Fragmentation

47. Managing Cluster Allocation
Linked
Each cluster has a pointer to the next cluster
Indexed
Single table has pointers to each of the clusters

48. Linked Blocks

49. Index Block

50. 8.6 Real-World Systems

51. 8.6 Real-World Systems Microsoft FAT Microsoft NTFS Linux Ext2, Ext3
Others

52. 8.6.1 MS FAT System Fat16 (FAT: file allocation table ) Fat32
MS-Dos, Windows 95
Max 2GB space for a FileSystem
Generally bad disk fragmentation
Fat32
Windows 98
Supported by Windows 2000, XP, 2003

53. The MS-DOS File System (1)
The MS-DOS directory entry

54. The Windows 98 File System (1)
Bytes
The extended MOS-DOS directory entry used in Windows 98

55. Cluster Sizes of FAT16 and FAT32
Drive Size
Default FAT16 Cluster Size
Default FAT32 Cluster Size
260 MB–511 MB
8 KB
Not supported
512 MB–1,023 MB
16 KB
4 KB
1,024 MB–2 GB
32 KB
2 GB–8 GB
8 GB–16 GB
16 GB–32 GB
> 32 GB

56. Windows FAT Table

57. 8.6.2. Windows NTFS File System
The NTFS file system (New Technology File System) is based on a structure called the "master file table" or MFT, which is able to hold detailed information on files. This system allows the use of long names, but, unlike the FAT32 system, it is case-sensitive, which means that is capable of distinguishing lower-case and upper-case letters.
Available on Windows 2000, XP, 2003
Maintains transaction log to recover after reboot
Support for file protection
Large (64 bit) cluster pointers
Allows small clusters
Avoids internal fragmentation

58. Windows NTFS File System
Master File Table: containing records about the files and directories of the partition. The first record, called a descriptor, contains information on the MFT (a copy of it is stored in the second record). The third record contains the log file, a file containing all actions performed on the partition. The following records, making up what is known as the core, reference each file and directory of the partition in the form of objects with assigned attributes.

59. File System Structure (1)
The NTFS master file table

60. File System Structure (2)
The attributes used in MFT records

61. File System Structure (3)
An MFT record for a three-run, nine-block file

62. 8.6.3 Linux Ext2 and Ext3 File System
Ext2 stands for second extended file system.
It was introduced in Developed by Rémy Card.
Maximum individual file size can be from 16 GB to 2 TB
Ext3
Ext3 stands for third extended file system.
It was introduced in Developed by Stephen Tweedie.
Starting from Linux Kernel ext3 was available.

63. Disk layout in classical UNIX systems
UNIX File System (1)
Disk layout in classical UNIX systems

64. UNIX File System (3)
The relation between the file descriptor table, the open file description

65. Directory entry fields.
UNIX File System (2)
Directory entry fields.
Structure of the i-node

66. Layout of the Linux Ex2 file system.
The Linux File System
Layout of the Linux Ex2 file system.