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Chapter 9: File-System Interface
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Mr. S. V. Viraktamath
Dept of E&CE,
SDMCT, Dharwad
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Chapter 10: File-System Interface
• File Concept
• Access Methods
• Directory Structure
• File-System Mounting
• File Sharing
• Protection
Introduction
• So far - Discussed
– CPU scheduling
– Process issues- Critical section problem
– Deadlock
– Main memory management
– Virtual memory
• File system is a most visible aspect of OS.
• It provides the mechanism for on-line storage of and access to both data and programs (OS+User)
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• File?
• Directories?
• Partitions?
• In this topic various aspects of files and the
major directory structures.
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• Storage Medias
• Magnetic disks, magnetic tapes, and optical
disks.
• Files are mapped, by the operating system,
onto physical devices.
• These storage devices are usually nonvolatile.
• A file is a named collection of related
information that is recorded on secondary
storage.
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Summary
• File system is a most visible aspect of OS.
• A file is an abstract data type defined and
implemented by the operating system.
• Collection of files Directories Partitions.
• Storage medias magnetic disks, magnetic
tapes, and optical disks
• Files are mapped by OS onto physical devices.
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File
• A file is the smallest allotment of logicalsecondary storage.
• Data cannot be written to secondary storageunless they are within a file.
• Data files may be numeric, alphabetic,alphanumeric, or binary.
• In general, a file is a sequence of bits, bytes,lines, or records.
• The information in a file is defined by itscreator
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• A file has a certain defined structure
according to its type.
• A text file is a sequence of characters
organized into lines
• Source file, Object file, executable file ?
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File Attributes
• A File name is usually a string of characters,
such as example1.c
• When a file is named, it becomes independent
of the process, the user, and even the system
that created it.
• A file has certain attributes
– Name -Type - Size
– Location - Protection
– Time, Date and user identification..
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Snapshot
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Snapshot of directory
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File operations
• The operating system can provide system calls
to create, write, read, reposition, delete, and
truncate files.
• Truncating a file: The user may want to erase
the contents of a file but keep its attributes.
• Other common operations - appending new
information to the end of an existing file and
renaming an existing file.
• Searching the directory for the file..
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Open files
• The OS keeps a small table containing
information about all open files (the open-file
table).
• File operation requested – no searching is
required
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Types of files
• A common technique for implementing file
types is to include the type as part of the file
name.
• The name is split into two parts-a name and an
extension, usually separated by a period
character. (Name and family name/address, USN)
• User and OS can understand which type of file
it is.
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• MS-DOS, a name can consist of up to eight
characters followed by a period and terminated
by an extension of up to three characters.
• A file with a .com, .exe, or .bat extension can
be executed.
• Assemblers expect source files to have an .asm
extension,
• The word processor expects its file to end with
a .doc extension..
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Common file types
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• File names
– Operatingsystem1
– ↓↨♥♣•◘¶☻▬↨
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Access Methods
• Files store information.
• This information must be accessed and read
into computer memory.
• Methods
– Sequential Access
– Direct Access
– Other Access Methods ..
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Access Methods
• Sequential Access
– Simplest access method
• Used : editors and compilers usually access
files in this fashion
• Bulk operations –reads and writes.
• read next; write next;
• no read after last write
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Sequential-access file.
• Sequential access is based on a tape model of a
file.
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Direct Access
• This method is based on a disk model of a file
• Disks allow random access to any file block
• A direct-access file allows arbitrary blocks to be
read or written.
• may read block 14, then read block 53, back to 7
• read n, where n is the block number, rather than
read next, and write n rather than write next.
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Other access method
• Can be built on top of a direct-access method.
• An index in the back of a book- like index -
contains pointers to the various blocks.
• first search the index, and then use the pointer
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Directory Structure
• The file systems of computers can beextensive.
• How many files may be there in u r system?
• Some systems store millions of files onterabytes of disk.
• First, disks are split into one or morepartitions-- files and directories reside.
• Partitions can also store multiple operatingsystems, allowing a system to boot and runmore than one.
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A typical file-system organization
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The operations - on a directory
• Search for a file- search directory for a file
• Create a file- new file added to directory
• Delete a file
• List a directory- able to list the files in a directory
• Rename a file- content changes
• Traverse the file system- the file can be copied
to tape.
• ………………………..
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Single-Level Directory
• The simplest directory structure
• All files are contained in the same directory
• As the number of files and users increases-limitations
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All Users-common folder
Two-Level Directory
• Create a separate directory for each user.
• Each user has her own user file directory(UFD)
• It lists only the files of a single user. It isolatesone user from another user.
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• When a user job starts or a user logs in, the
system's master file directory (MFD) is
searched.
• The MFD is indexed by user name or account
number, points to the UFD for that user. (C pgms)
To name a particular file uniquely we must give
both user name and file name.
System files stored in directory user 0.
Can have the same file name for different user
Efficient searching
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Tree-Structured Directories
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MS DOS –structures files as a tree
Tree is most common directory structure.
Tree has root directory.
Path name is path from root, through all
subdirectories to a specified file.
A directory contains a set of files or
subdirectories.
• Each user has a current directory.
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• Absolute path name: It begins at the root and follows a path down to the specified file, giving the directory names on the path.
• A relative path name:
Defines a path from the current directory.
Ex. Current directory root/spell/mail then
• Absoluteroot/spell/mail/prt/first
• Relative prt/first
• If directory is empty, it can be deleted. (MSDOS)
• Unix rm command removes /deletes a directory.
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Acyclic-Graph Directories
• Consider two programmers who are working
on a joint project.
• There may be common subdirectory between
them.
• An acyclic graph allows directories to have
shared subdirectories and files.
• The same file or subdirectory may be in two
different directories.
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• A tree structure prohibits the sharing of files or
directories.
• An acyclic graph allows directories to have
shared subdirectories and files.
• An acyclic graph, that is, a graph with no
cycles, is a natural generalization of the tree
structured directory scheme.
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• With a shared file, only one actual file exists,
so any changes made by one person are
immediately visible to the other.
• One approach to implementing shared files is
simply to duplicate all information about them
in both sharing directories.
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General Graph Directory
• when we add links to an existing tree-structured directory, the tree structure isdestroyed resulting in a simple graphstructure.
• If cycles are allowed to exist in the directory,we likewise want to avoid searching anycomponent twice.
• A poorly designed algorithm might result in an infinite loop continually searching through the cycle and never terminating.
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General Graph Directory
• How do we guarantee no cycles?
– Allow only links to file not subdirectories
– Every time a new link is added - use a cycle
detection algorithm to determine whether it is OK
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Allocation Methods
• An allocation method refers to how disk
blocks are allocated for files:
– Contiguous allocation
– Linked allocation
– Indexed allocation
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Contiguous Allocation
• Each file occupies a set of contiguous blocks
on the disk.
• Simple – only starting location (block #) and
length (number of blocks) are required
• Random access – possible.
• Wasteful of space (dynamic storage-allocation
problem)
• Files cannot grow
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Contiguous Allocation of Disk Space
• Accessing block b + 1 after block b normallyrequires no head movement.
• When head movement is needed, it is only onetrack.
• The number of disk seeks required foraccessing contiguously allocated files isminimal.
– The amount of time that a disk drive's head takesto move to a specific location on a disk.
• Both sequential and direct access can besupported by contiguous allocation.
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Disadv
• Difficulty is finding space for a new file.
• First fit and best fit are the most common
strategies used to select a free hole from the set
of available holes.
• Algorithms suffer from the problem of
external fragmentation. Why?
• Used in
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• Older microcomputer systems used contiguous allocation on floppy disks.
• Run a repacking routine- copy to anotherfloppy or tape.
• Another problem with contiguous allocation is determining how much space is needed for a file.
• If we allocate too little space to a file – it may be extended
• If you allocate too much space -inefficient.
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Linked Allocation
• Linked allocation solves all problems ofcontiguous allocation.
• With linked allocation, each file is a linked listof disk blocks; the disk blocks may bescattered anywhere on the disk.
• The directory contains a pointer to the first and last blocks of the file.
• Each block contains a pointer to the next block
• These pointers are not made available to the user. (Game)
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Pointers –adr is not correct
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• To create a new file, we simply create a newentry in the directory.
• This pointer is initialized to nil (the end-of-listpointer value) to signify an empty file.
• There is no external fragmentation with linkedallocation
• The size of a file does not need to be declaredwhen that file is created.
• A file can continue to grow as long as freeblocks are available.
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Dis adv
• It can be used effectively only for sequential-
access files.
• To find the ith block of a file, we must start at
the beginning of that file.
• Another disadv-the space required for the
pointers.
• If a pointer requires 4 bytes out of a 512-byte
block, then 0.78 percent of the disk used.
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• Solution - to collect blocks into multiples,
called clusters and to allocate the clusters
rather than blocks.
• Yet another problem of linked allocation is
reliability.
• If a pointer were lost or damaged.
• Partial solutions are to use doubly linked lists.
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Indexed Allocation
• Linked Allocation –Adv?
– No external fragmentation & Size declaration pblm
• Linked allocation Disadv?
– No direct Access
• IndexedNo External fragmentation & Direct
access
• All pointers brought into one location called
index block.
• Each file has its own index block.
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• How to access 3rd block of a file?
• The directory contains the address of the index
block.
•
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• Indexed allocation does suffer from wastedspace Pointer overhead is more.
• Ex. a file of only one or two blocks.
• If the index block is too small - it will not beable to hold enough pointers for a large file.
• Solution:
– Linked Scheme :link together several index blocks
– Multilevel index: first level index block to point to a set of second-level index blocks, which in turn point to the file blocks.
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Free-Space Management
• Disc space limited-reused.
• The system maintains a free-space list.
• To create a file -search the free-space list-
there- removed from the free-space list.
• File deleted -its disk space is added to the free-
space list.
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Bit Vector
• The free-space list is implemented as a bit map
or bit vector.
• Each block is represented by 1 bit. If the block
is free, the bit is 1.
• Blocks 2, 3,4,5, 8, 9, 10, 11, 12, 13, 17, 18, 25,
26, and 27 are free, and the rest of the blocks
are allocated.
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• The main advantage-relatively simplicity.
• Disadvantage
– Bit vectors are inefficient unless the entire vector
is kept in main memory.
– A 1.3-GB disk with 512-byte blocks would need a
bit map of over 332 KB to track its free blocks
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Linked List
• Linked Allocation?
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Linked List
• Another approach is to link together all the
free disk blocks.
• Keeping a pointer to the first free block.
• First block contains a pointer to the next free
disk block, and so on.
• Ex. keep a pointer to block 2, it contain a
pointer to block 3, which would point to block
4, which would point to block 5.
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Linked free space list on disk
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• Disadvantages
– we must read each block
• Grouping:
– A modification of the free-list approach is to store
the addresses of n-1 free blocks in the first free
block.
– The last block contains the addresses of another nfree blocks, and so on.
– Adv to compared to linked as most of the free
blocks addresses are available in one block.
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• Counting:
– WKT several contiguous blocks may be allocated
or freed simultaneously (in contiguous-allocation)
– Rather than keeping a list of n free disk addresses
keep the address of the first free block and the
number n.
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End of Topic
ಆನಂದಾಶ್ರಮಮತ್ತು ಮಹಿಷಿ ಟ್ರಸ್ಟದಲಿ್ಲ ಎಸ್ ಡಿ ಎಮ್ ಕನನಡ ವೆೇದಿಕೆಯ ಸ್ಹಯೇಗದೆ ಂದಿಗೆಹಮ್ಮಿಕೆ ಂಡಿರತವ ಕಿರತ ಕಾಣಿಕೆ ಹಾಗ ಸ್ಂಗೇತ್ ಕಾಯಯಕರಮದಲ್ಲಿ ಧ್ವನಿ ತ್ಂಡ ನಡೆಸಿದ
ಸ್ಂಗೇತ್ ಕಾಯಯಕರಮ ಅಲಿ್ಲಯ ಜನರಮನಗೆದತು ಅವರನತನ ಖತಷಿಯಾಗಸಿತ್ತ.
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• Daughter of Pandit Mallikarjun Mansoor
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End of Topic
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Chandrakant Kanavi
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• Prof. Shakuntala
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End of Topic
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