Transcript File Management

Understanding Operating Systems
Fifth Edition
Chapter 8
File Management
The File Manager
• File Manager is also called the File management
system and is the software responsible for:
– Creating, deleting, modifying, controlling access to
files
• Provides support for libraries of programs to online
users, for spooling operation, and for interactive
computing
Understanding Operating Systems, Fifth Edition
2
Responsibilities of the File Manager
• Four tasks
– Keep track of where each file is stored
– Implement a policy that will:
• Determine where and how files are stored
• Efficiently use available storage space
• Provide efficient file access
– Allocate each file when a user has been cleared for
access to it, then record its use
– File deallocation
• File returned to storage
• Communicate file availability to others waiting for it
Understanding Operating Systems, Fifth Edition
3
Responsibilities of the File Manager
(continued)
• File Manager’s Policy determines:
– File storage location
– System and user access files via device-independent
commands
• Who will have access to which file. This depends on
two factors:
• Flexibility of access to information (Factor 1)
– Shared files
– Providing distributed access
– Allowing users to browse public directories
Understanding Operating Systems, Fifth Edition
4
Responsibilities of the File Manager
(continued)
• Subsequent protection (Factor 2)
– Protect files against system malfunctions
– Security checks
• Account numbers, passwords, lockwords
• File allocation
– Activate secondary storage device, load file into
memory, update records
• File deallocation
– Update file tables, rewrite file (if revised), notify
waiting processes of file availability
Understanding Operating Systems, Fifth Edition
5
Definitions
• Field
– Group of related bytes (e.g. last_name, first_name,
age) that can be identified by the user with a name,
type, and size.
• Record
– Group of related fields
• File
– Group of related records that contains information to
be used by specific application programs to generate
reports. This type of file contains data and is
sometimes called a flat file because it has no
connections to other files and has no dimensionality.
Understanding Operating Systems, Fifth Edition
6
Definitions (continued)
• Databases
– Groups of related files
– Interconnected at various levels
• Give users flexibility of access to stored data
• Program files
– Contain instructions
• Data files
– Contain data
• Directories
– Are special files with listings of filenames and their
attributes
Understanding Operating Systems, Fifth Edition
7
Definitions (continued)
Understanding Operating Systems, Fifth Edition
8
Interacting with the File Manager
• User interacts with the File Manager with commands that are
either:
– Embedded in program
• OPEN, CLOSE, READ, WRITE, MODIFY
– Submitted interactively
• CREATE, DELETE, RENAME, COPY
• Device independent
– Physical location knowledge not needed
• Cylinder, surface, sector
– Device medium knowledge not needed
• Tape, magnetic disk, optical disc, flash storage
– Network knowledge not needed
Understanding Operating Systems, Fifth Edition
9
Interacting with the File Manager
(continued)
Understanding Operating Systems, Fifth Edition
10
Interacting with the File Manager
(continued)
• Each of the logical commands (READ, WRITE etc.) can be
broken into lower-level signals.
– Example: READ
• Move read/write heads to record cylinder
• Wait for rotational delay (sector containing record passes
under read/write head)
• Activate appropriate read/write head and read record
• Transfer record to main memory
• Send flag indicating free device for another request
• File Manager does all of this and performs error checking and
correction
– No need for error-checking code in programs
Understanding Operating Systems, Fifth Edition
11
Typical Volume Configuration
• Secondary storage unit can be removable (such as
CDs, DVDs etc) or non-removable (such as hard
disks). Each storage unit is considered a volume.
– A multifile volume contains many files
– Multivolume files
• These are extremely large files spread across several
volumes
• Each volume is given a name
– This is managed by the File manager and it writes this
descriptive information in an easily accessible place
• Innermost part of CD, beginning of tape, first sector of
outermost track
Understanding Operating Systems, Fifth Edition
12
Typical Volume Configuration
(continued)
Understanding Operating Systems, Fifth Edition
13
Typical Volume Configuration
(continued)
• Master file directory (MFD)
• Is stored immediately after volume descriptor
• MFD lists:
– Names and characteristics of every file in volume
• File names (program files, data files, system files)
– Any subdirectories
• If supported by file manager
– The remainder of the volume
• Is used for file storage
Understanding Operating Systems, Fifth Edition
14
Typical Volume Configuration
(continued)
• Single directory per volume
– Supported by early operating systems
• Disadvantages
–
–
–
–
–
Long search time for individual file
Directory space filled before disk storage space filled
Users cannot create subdirectories
Users cannot safeguard their files
Each program needs unique name
• Even those serving many users
Understanding Operating Systems, Fifth Edition
15
About Subdirectories
• Newer File Managers
– Create MFD for each volume
• Contains file and subdirectory entries
– Improvement over single directory scheme
• Problems remain: users unable to logically group files by
creating their own subdirectories.
• Subdirectory
– Is created when a user opens an account in the computer
system
– Is treated as a file
• Though flagged in MFD as subdirectory
• Has unique properties since its records are filenames
pointing to files
Understanding Operating Systems, Fifth Edition
16
About Subdirectories (continued)
• File managers today
– Users create own subdirectories (folders)
• Related files grouped together
– Implemented as upside-down tree
• Efficient system searching of individual directories
• May require several directories to reach file
Understanding Operating Systems, Fifth Edition
17
About Subdirectories (continued)
Understanding Operating Systems, Fifth Edition
18
About Subdirectories (continued)
• File descriptor
– Filename: ASCII code
– File type: organization and usage
• System dependent
– File size: for convenience
– File location
• First physical block identification
–
–
–
–
Date and time of creation
Owner
Protection information: access restrictions
Record size: fixed size, maximum size
Understanding Operating Systems, Fifth Edition
19
File-Naming Conventions
• Filename components
– Relative filename and extension
• Complete filename (absolute filename)
– Includes all path information
• Relative filename
–
–
–
–
Name without path information
Appears in directory listings, folders
Provides filename differentiation within directory
Varies in length
• One to many characters
• Operating system specific
Understanding Operating Systems, Fifth Edition
20
File-Naming Conventions (continued)
• Extensions
– Appended to relative filename
• Two to three characters
• Separated by period
• Identifies file type or contents
– Example
• BASIA_TUNE.MPG
– Unknown extension
• Requires user intervention
Understanding Operating Systems, Fifth Edition
21
File-Naming Conventions (continued)
Understanding Operating Systems, Fifth Edition
22
File-Naming Conventions (continued)
• Operating system specifics
– Windows
• Drive label and directory name, relative name, and
extension
– Network with Open VMS Alpha
• Node, volume or storage device, directory,
subdirectory, relative name and extension, file version
number
– UNIX/Linux
• Forward slash (root), first subdirectory, subsubdirectory, file’s relative name
Understanding Operating Systems, Fifth Edition
23
File Organization
• Arrangement of records within files
• All files composed of records
• Modify command
– Request to access record within a file
Understanding Operating Systems, Fifth Edition
24
Record Format
• Fixed-length records
– Easiest to access directly
– The record size is critical
– Ideal for data files
• Variable-length records
–
–
–
–
Difficult to access directly
No empty storage space and no character truncation
File descriptor stores record format
Used with files accessed sequentially
• Text files, program files
– Used with files using index to access records
Understanding Operating Systems, Fifth Edition
25
Record Format (continued)
Understanding Operating Systems, Fifth Edition
26
UNIX File Management (from Chapter 13)
• Three file types
– Directories
– Ordinary files
– Special files
• Each enjoys certain privileges
• Directories
– Are files that maintain the hierarchical structure of file
system
– Users allowed to read information in directory files
– Only system allowed directory file modification
Understanding Operating Systems, Fifth Edition
27
UNIX File Management (continued)
• Ordinary files
– Are those in which users store information
– Their protection is based on user requests and are
related to the read, write, execute, delete functions
performed on a file
• Special files
–
–
–
–
Device drivers providing I/O hardware interface
Appear as entries in directories
Part of file system (most in /dev directory)
The name of the special file indicates type of device
association (e.g. /dev/hd, /dev/lp)
Understanding Operating Systems, Fifth Edition
28
UNIX File Management (continued)
• Files stored as sequences of bytes
– No structure imposed
• Text files
– Character strings
• Lines delimited by line feed or new line character
• Binary files
– Sequences of binary digits (bits)
• Grouped into words as they appear in memory during
program execution
• Structure of files
– Controlled by programs using them and not by UNIX
Understanding Operating Systems, Fifth Edition
29
UNIX File Management (continued)
• Organizes disk into blocks of 512 bytes each
• Divides disk into four basic regions
– First region (address 0): reserved for booting
– Second region: contains disk size and other regions’ boundaries
– Third region includes: list of file definitions called the “i-list”,
which is a list of file descriptors (called “i-nodes”), one for each
file. The position of an i-node is called an “i-number” and this
uniquely identifies a file.
– Remaining region: free blocks available for file storage
• Files stored in contiguous empty blocks
– Simple allocation and no need to compact
Understanding Operating Systems, Fifth Edition
30
UNIX File Management (continued)
• “i-node”
• Each entry in i-list called an “i-node” (or inode)
– Contains 13 disk addresses
• Contains specific file information
–
–
–
–
–
Owner’s identification
Protection bits, physical address, file size
Time of creation, last use, and last update
Number of links
File type
• Directory, ordinary file, or special file
Understanding Operating Systems, Fifth Edition
31
UNIX File Management (continued)
UNIX File Naming Conventions
• Case-sensitive filenames
• 255 character length
• While UNIX does not impose any file naming conventions,
system programs such as compilers expect files to have
suffixes (“extensions”).
- E.g. prog1.c or prog1.java
• Supports hierarchical tree file structure
– Root directory identified by slash (/)
– The names of other directories are preceded by the slash (/),
which is used as a delimiter
E.g. /programs/pay/checks
where checks is the actual file (see figure 13.8)
The first slash indicates that this is an absolute path name that
starts at the root directory. A relative path name would be
pay/checks.
Understanding Operating Systems, Fifth Edition
33
UNIX File Naming Conventions (continued)
Understanding Operating Systems, Fifth Edition
34
UNIX File Naming Conventions (continued)
• Path name rules
– If a path name starts with a slash, the path starts at
the root directory
– Path name can be either:
• One name or a list of names separated by slashes
• Last name on a list is the name of the file requested
– Using two periods (..) in path name
• Moves you upward in hierarchy (closer to root). This is
the only way to go up the hierarchy
• All other path names go down tree
– Spaces not allowed within path names
Understanding Operating Systems, Fifth Edition
35
UNIX Directory Listings
• Table 13.2 shows a “long listing” of files in a directory with
eight pieces of information for each file
• First column
– Shows file type and access privileges for each file
• First character: nature of file or directory (‘d’ indicates a
directory and ‘-’ indicates an ordinary file)
• Next three characters: show the access privileges granted to
the file owner (‘r’ indicates read, ‘w’ indicates write, and ‘x’
indicates execute)
• Next three characters: show the access privileges granted to
other members of the user’s group (e.g. students in a class
could be a group)
• Last three characters: show the access privileges granted to
users at large (system-wide)
Understanding Operating Systems, Fifth Edition
36
UNIX Directory Listings (continued)
• Second column
– Indicates number of links (number of aliases)
• Referring to same physical file
• Aliases (links)
– Important UNIX feature: support file sharing
• Several users work together on same project
– Shared files appear in different directories belonging
to different users
– Filename: may be different from directory to directory
– Eventually number will indicate when file no longer
needed: can be deleted
Understanding Operating Systems, Fifth Edition
37
UNIX Directory Listings (continued)
Understanding Operating Systems, Fifth Edition
38
UNIX Data Structures
• File descriptors divided into parts
– Hierarchical directories
• Contain filename and i-number
• Pointer to another location: i-node
– i-node
• Contains rest of information
– i-nodes stored in reserved part of device
• Where directory resides
– i-node has 13 pointers (0–12)
Understanding Operating Systems, Fifth Edition
39
UNX Data Structures (continued)
Understanding Operating Systems, Fifth Edition
40
UNIX Data Structures (continued)
• When file created
– i-node allocated to it
– Directory entry with filename and i-node number
created
Understanding Operating Systems, Fifth Edition
41
UNIX Data Structures (continued)
• When file linked
– Directory entry created with new name
– Original i-node number and link-count field in the inode incremented by one
• When shared file deleted
– Link-count field in i-node decremented by one
– When count reaches zero
• Directory entry erased
• Deallocate all disk blocks, along with its i-node entry in
the disk i-list
Understanding Operating Systems, Fifth Edition
42