1. File Management Systems
Chapter 12
File Management Systems

2. Chapter goals
Describe the components and functions of a file management system
Compare the logical and physical organization of files and directories
Explain how secondary storage locations are allocated to files and describe the data structures used to record those allocations

3. Chapter goals cont.
Describe file manipulation operations, including open, close, read, delete, and undelete operations
List access controls that can be applied to files and directories
Describe security, backup, recovery, and fault tolerance methods and procedures

4. File Management Systems
FMS is implemented in layers like the OS
Command layer or application program
File control
Storage I/O control
Storage devices

5. File Man. Systems Layers

6. FMS Layers Storage devices – corresponds with hardware layer of OS
Interacts with the bus and with operating system device drivers to transfer data between storage devices and memory

7. FMS layers
Storage I/O control – accesses storage locations and manages data movement between storage devices and memory
Part of OS kernel
Its software modules include
Device drivers for each storage device or device controller
Interrupt handlers
Buffers and cache managers

8. FMS layers cont.
File Control – provides a set of service functions for manipulating files and directories
Processes service calls from users and applications
Maintains directory and storage allocation data structures used to locate files and their associated physical locations
Command layer or application program – users perform common file management functions such as copying, moving and renaming

9. Logical vs. physical view
Logical Storage Views – viewed by users are a collection of files organized within directories and storage volumes
Physical Storage Views – a collection of physical storage locations organized as a linear address space

10. Logical vs. physical view

11. Logical vs. physical view of a file
File is subdivided into records
Record usually contains information about a single customer, thing such as a product in inventory, or an event
Records are divided into fields
Fields are individual units of data

12. Logical vs. physical
Logical file structure is independent of its physical implementation
Logical file structure “ignores”
Physical storage allocations – records can be stored in separate file locations
Data access methods
Data encoding methods

13. Logical structure of data file

14. File content and type
A file is a collection of data created by an applications
The format of that information is called the file type
A file can store many different data types including text, numbers, complex data structures, and executable instructions
Modern file management systems provide a framework to support additional file types

15. File type File type normally is declared when a file is created
In the UNIX file management system, the file type is stored within the directory. In the Windows file management system, the file type is declared through the extension

16. Registered Windows File Types
In windows the extension of a file can be associated (connected) to a specific application
When you open a file associated with an application, that application is started and the file is opened in the associated application
Show associated file types – control panel, folder options

17. Directory content and structure
Files are organized into directories
Some directories are created and maintained by software
Windows directories
Directories associated with applications
User can also create and maintain directories

18. Directory content Directories in windows are organized in a hierarchy
Directory information includes:
Name
File type
Location
Size
Ownership
Access controls
Time stamp(s)
Show directory information for C drive

19. Directory Content and Structure
Typical file ownership permissions are:
Create
Read
Update
delete
Time stamps include:
When the file was created
When the file most recently was read
When the file most recently was written
When the file last was backed up

20. Hierarchical Directory Structure
Windows directory structure is hierarchical
Directories can contain other directories (called sub-directories)
Directories can not have more than one parent
Sometimes called a tree structure (draw picture)

21. Hierarchical directory structures

22. Storage allocation
Users and applications programs continually create and change files
When files are created they must be given space in storage
When files grow they must be given additional space
When files shrink unneeded space must be released

23. Allocation units
Allocation unit is smallest unit of space that can be allocated to a file
Allocation units cannot be smaller than system data transfer unit
Data transfer unit is called a block
Block sizes range from 512 bytes to 4 KB in multiples of 512 bytes

24. Allocation units cont.
Is a multiple of block size, i.e. equal to 4 blocks, 8 blocks, etc.
Size of allocation unit is optimized to use space efficiently

25. Storage allocation tables
FMS maintains a table of storage allocation units
Records which units belong to which file
Records which units are available (free)
In windows this table called File Allocation Table or FAT

26. Sample allocations of files
The next three slides show 3 files, how their storage is allocated, and how FMS records location of allocation units
Storage units are said to be “chained together” using pointer
Each unit contains reference (pointer) to next unit in the list

27. Directory listing of 3 files

28. Where 3 files are stored

29. How FMS finds files

30. Blocking and buffering
A logical record is a collection of data items, or fields, that is accessed by an application program as a single unit
A physical record is the unit of storage transferred between the device controller and memory in a single operation

31. Blocking and buffering
Most of the time the logical size of a record does not equal the physical allocation unit
When several logical records are grouped
within physical records that is called blocking
Individual logical records grouped together into one unit called a block

32. Blocking and buffering cont.
Simplifies data transfer between drive and memory
Then OS must extract individual records from the allocation unit block
If a physical record contains just one logical record, then the file is said to be unblocked

33. Blocking records

34. Buffer
Buffer is storage area in memory where blocks of records from drive are copies
Buffer is used to extract individual records from a block

35. Using buffers

36. Operations on files
There are setup requirements that need to be executed whenever a file is first requested by an application and when that file is not longer needed
These operations are called
File open
File close

37. File open
When an application requests a file, the FMS must do the follow:
The file must be located on the storage device
Ensure that application has right to access this file
Allocates one or more buffers
Updates internal table of open files

38. File close
When application sends request to close a file, FMS does the following:
Flushing the program’s I/O buffers to secondary storage
De-allocate buffer memory
Update directory entry time stamp
Update file open table

39. Delete and Undelete Operations
In most file management systems, files are not removed immediately from secondary storage when they are deleted
The file’s storage allocation units are marked as free and its directory entry is marked as unused
A user might be able to use the undelete operation to recover the file
Recyling bin in windows allows files to be undeleted

40. Access controls
A File Management System helps prevent loss, corruption and unauthorized access to files
The operating system is used to identify and authenticate users and their processes
The file access is authenticated through id’s and passwords

41. Access control For Example:
UNIX defines three access control types:
Read
Write
Execute
Internet web sites – you can read pages but not change them (unless you are a hacker)

42. FMS Backup options
Full Backup – the FMS copies all files and directories for an entire storage volume
Incremental Backup – only the files that have been modified are archived
Differential Backup – only the changed portions of the files are archived

43. Windows backup
Programs accessories system tools

44. Summary
The file management system (FMS), usually a part of the operating system, manages all aspects of user and program access to secondary storage
With directories, users can organize the thousands of files stored in a typical computer system
Secondary storage units are divided into allocation units, which are typically a few kilobytes in size

45. Summary cont. The FMS allocates buffers to support program file I/O
The FMS enforces access controls when accessing files on behalf of a user or program
FMSs provide utilities to make backup copies of files and directories and to recover them if needed