1. Chapter 12 File Management
Overview
File organisation and Access
File Directories
File Sharing
Record Blocking

2. Files Files are the central element to most applications
file as an input to applications
file as an output for long-term storage and for later access
Desirable properties of files:
Long-term existence
Controlled sharing between processes
Structure that is convenient for particular applications

3. File Structure Fields and Records
Basic element of data
e.g., student’s last name
Contains a single value
Characterized by its length and data type
Records
Collection of related fields
e.g., a student record
Treated as a unit

4. File Structure File and Database
Collection of similar records
Treated as a single entity and may be referenced by name
Access control restrictions usually apply at the file level
Database
Collection of related data
Explicit relationships exist among elements
Consists of one or more files

5. A Big Picture
individual block I/O requests must be scheduled for optimizing performance
How to organize records as a sequence of blocks for I/O?
How to organize records in a file and access a particular record in a file?
How to identify and locate a selected file?
How to enforce user access control in shared systems?

6. Roadmap File organisation and Access Overview File Directories
File Sharing
Record Blocking

7. File Organization
The basic operations that a user or application may perform on a file are performed at the record level
The file is viewed as having some structure that organizes the records
File organization refers to the logical structuring of records
Determined by the way in which files are accessed (access method)

8. Criteria for File Organization
Important criteria include:
Short access time
Ease of update
Economy of storage
Simple maintenance
Reliability

9. Criteria for File Organization
Priority will differ depending on the use
For batch mode file processing, rapid access for retrieval of a single record is of minimal concern
These criteria may conflict
Use of indexes (conflict with economy of storage) can be a primary means of increasing the speed of access to data

10. The Pile Data are collected in the order they arrive
No structure
Purpose is to accumulate a mass of data and save it
Records may have different fields
field should be self-describing (field name + value)
field length should be known (delimiters, subfield or default for a field type)

11. The Pile Record access is by exhaustive search
Used when data are collected and stored prior to processing or data are not easy to organize
 Uses space well when data vary in size and structure
 Adequate for exhaustive searches
 Easy to update
 Unsuitable for most applications

12. The Sequential File Fixed format used for records
Records are of the same length
same number of fixed-length fields in a particular order
Only the values of fields need to be stored
Field name and length are attributes of the file structure

13. The Sequential File Key field
Uniquely identifies the record
Records are stored in key sequence
 Optimal for batch applications if they involve the processing of all the records
 Easily stored on tape and disk
 Poor performance for interactive applications
considerable processing and delay due to the sequential search of the file for a key match

14. Indexed Sequential File
An index is added to support random access
An index record contains a key field and a pointer into the main file
The index is a sequential file
For searching
Search the index to find the highest key value that is equal to or precedes the desired key value
Search continues in the main file at the location indicated by the pointer

15. Indexed Sequential File Example
Consider searching a particular key value in a sequential file with 1 million records
without index
requires on average one-half million record accesses
with an index containing 1000 entries with the keys in the index evenly distributed over the main file
requires on average 500 accesses to the index file accesses to the main file

16. Indexed Sequential File
An overflow file is added
A new record is added to the overflow file and is located by following a pointer from its predecessor record
The indexed sequential file is occasionally merged with the overflow file in batch mode
Greatly reduces the time required to access a single record, without sacrificing the sequential nature.

17. Indexed File Records are accessed only through their indexes
no restriction on the placement of records
allows variable-length records
Uses multiple indexes for different key fields
An exhaustive index contains one entry for every record in the main file
A partial index contains entries to records where the field of interest exists

18. Indexed File
When a new record is added to the main file, all of the index files must be updated.
Used mostly in applications where
timeliness of information is critical and
data are rarely processed exhaustively
examples: airline reservation systems and inventory control systems

19. Roadmap File Directories Overview File organisation and Access
File Sharing
Record Blocking

20. File Directory Contains information about files
Attributes
Location
Ownership
Directory itself is a file owned by the operating system

21. Directory Elements Basic Information Address Information
File name: must be unique
File type: e.g., text, binary
File organization
Address Information
Volume: device on which file is stored
Starting address: e.g., cylinder, track on disk
Size used: in bytes, words or blocks
Size allocated: maximum size of the file

22. Directory Elements Access Control Information Usage Information
Owner: able to grant/deny access to other users and to change these privileges
Access information: e.g., user’s name and password for each authorized user
Permitted actions: controls reading, writing, executing, transmitting over a network
Usage Information
Date Created, Identity of Creator, Date Last Read Access, Identity of Last Reader, Date Last Modified

23. Hierarchical, or Tree-Structured Directory
Master directory with user directories underneath it
Each user directory may have subdirectories and files as entries
Each directory and subdirectory can be organized as a sequential file

24. Hierarchical, or Tree-Structured Directory
 Easily enforce access restriction on directories.
 Easily organize collections of files.
 Minimize the difficulty in assigning unique names.

25. Naming
The tree structure allows users to find a file by following a path from the root or master directory down various branches until the file is reached
The series of directory names, culminating in the file name itself, constitutes a pathname for the file
Duplicate filenames are possible if they have different pathnames

26. Naming
Usually an interactive user or a process is associated with a current or working directory
Files are referenced relative to the working directory unless an explicit full pathname is used

27. Roadmap File Sharing Overview File organisation and Access
File Directories
File Sharing
Record Blocking

28. File Sharing
In multiuser system, there is almost always a requirement for allowing files to be shared among a number of users
Two issues
Access rights
Management of simultaneous access

29. Access Rights
A wide variety of access rights have been used by various systems
often as a hierarchy, with each right implying those that precede it.
None
User may not know the existence of file by not allowing to read the user directory that includes this file
Knowledge
User can only determine that the file exists and who its owner is

30. Access Rights cont… Execution Reading Appending
The user can load and execute a program but cannot copy it, e.g., proprietary programs
Reading
The user can read the file for any purpose, including copying and execution
Appending
The user can add data to the file but cannot modify or delete any of the file’s contents

31. Access Rights cont… Updating Changing protection Deletion
The user can modify, delete, and add to the file’s data.
Changing protection
User can change access rights granted to other users
Deletion
User can delete the file

32. User Classes Access can be provided to different classes of users
Owner: usually the files creator, has full rights and may grant rights to others
Specific users: individual users who are designated by user ID
User groups: a set of users identified as a group
All: all users who have access to this system

33. Simultaneous Access
When access is granted to append or update a file to more than one user, the OS or file management system must enforce discipline
User may lock the entire file or individual records during update
Mutual exclusion and deadlock are issues for shared access, ref. readers/writers problem

34. Roadmap Record Blocking Overview File organisation and Access
File Directories
File Sharing
Record Blocking

35. Blocks and records
Records are the logical unit of access of a structured file
Blocks are the unit for I/O with secondary storage
For I/O to be performed, records must be organized as blocks.
Three methods of blocking are common
Fixed length blocking
Variable length spanned blocking
Variable-length unspanned blocking

36. Fixed Blocking
Fixed-length records are used, and an integral number of records are stored in a block
Unused space at the end of a block is internal fragmentation
Common for sequential files with fixed-length records

37. Fixed Blocking

38. Variable Length Spanned Blocking
Variable-length records are used and are packed into blocks with no unused space
Some records may span multiple blocks
Continuation is indicated by a pointer to the successor block
 Efficient for storage and does not limit the size of records

39. Variable Blocking: Spanned
 Difficult to implement
 Records that span two blocks require two I/O operations

40. Variable-length unspanned blocking
Uses variable length records without spanning
 Wasted space in most blocks because of the inability to use the remainder of a block if the next record is larger than the remaining unused space
 Limits record size to the size of a block

41. Variable Blocking: Unspanned

42. Revisit the Big Picture
Records must be organized as a sequence of blocks for output and unblocked after input
individual block I/O requests must be scheduled for optimizing performance
User views the file as having some structure that organizes the records; different access methods reflect different file structures
Describes the location of all files plus their attributes
Only authorized users are allowed to access particular files in particular ways