File System

1. File-System Interface
File Concept
Access Methods
Directory Structure
File-System Mounting
File Sharing
Protection

2. 1. File Concept
• A file is a named collection of related information that is
recorded on secondary storage
• User's perspective, a tile is the smallest allotment of
logical secondary storage i.e., data cannot be written to
secondary storage unless they are within a file
• Types of information may be stored in a file
– Data – numeric , character, binary
– Program – Source, Object, Executable
• A file has a defined structure depends on its type
– A Source file is a sequence of subroutines and functions
– An Object file is a sequence of bytes
– An Executable file is a series of code sections
Loganathan R, CSE, HKBKCE 2

3. 1. File Concept
1.1 File Attributes
Vary from one OS to another, but consists:
• Name – only information kept in human-readable form
• Identifier – unique tag (number) identifies file within
file system (non-human-readable name)
• Type – needed for systems that support different types
• Location – pointer to file location on device
• Size – current file size
• Protection – controls who can do reading, writing,
executing
• Time, date, and user identification – data for
protection, security, and usage monitoring
• Information about files are kept in the directory
structure, which is maintained on the disk
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4. 1. File Concept Cntd…
1.2 File Operations
File is an abstract data type OS System calls for operations
• Creating a file – 2 steps: Find space in the file system for the file and an
entry in the directory
• Writing a file - name of the file and the information to be written to the file
• Read - name of the file and where (in memory)the next block of the file,
read pointer to the location of next read, per-process current file- position
pointer
• Reposition within file - current-file-position pointer is repositioned to a
given value, also known as a file seek
• Deleting a file - release all file space and erase the directory entry
• Truncating a file - all attributes to remain unchanged—except for file length
• appending new information to the end of an existing file
• renaming an existing file
• copy of a file
• Open(Fi) – search the directory structure on disk for entry Fi, and move the
content of entry to memory
• Close (Fi) – move the content of entry Fi in memory to directory structure on
disk
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5. 1. File Concept Cntd…
Open file table
• Contains information about all open files
– File pointer: pointer to last read/write location, per process that has the file open
– File-open count: counter of number of times a file is open – to allow removal of data from
open-file table when last processes closes it
– Disk location of the file: cache of data access information
– Access rights: per-process access mode information
File locks
• Allow one process to lock a file and prevent other processes from gaining access to
it.
• Useful for files that are shared by several processes(eg. System log file)
• A shared lock is similar to a reader lock , several processes can acquire the lock
concurrently
• An exclusive lock behaves like a writer lock only one process at a time can acquire
• A Mandatory lock– once a process acquires an exclusive lock, the OS will prevent
any other process from accessing the locked file
• An Advisory lock– processes can find status of locks and decide what to do
Loganathan R, CSE, HKBKCE 5

6. 1. File Concept Cntd…
1.3 File Types
• OS should recognize and
support file types, so that
it can then operate on the
file in reasonable ways
• 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, separated by a
period character
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7. 2. Access Methods
2.1 Sequential Access
• Information in the file is processed in order, one record after the other
• read next—reads the next portion of the file and automatically advances
a file pointer
• write next—appends to the end of the file
• reset - to the beginning
• forward or backward n records
Loganathan R, CSE, HKBKCE 7

8. 2. Access Methods Contd…
2.2 Direct Access / relative access
• A file is made up of fixed length logical records that allow programs to read
and write records rapidly in no particular order
• File operations modified to include the block number as a parameter
• Block number provided by the user to the OS is a relative block number
• A relative block number is an index relative to the beginning of the file
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9. 2. Access Methods Contd…
2.3 Other Access Methods
• Built on top of a direct-access method
• Example of Index and Relative Files
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10. 3. Directory Structure
A collection of nodes containing information about all file
3.1 Storage Structure
• Both the directory structure and the files reside on disk
• Parts of a disk(partitions, slices, or minidisks) is used for a file system and other
parts for other things
• The parts can also be combined to form larger structures known as volumes
• A device directory or volume table of contents commonly
• Known as a directory records information—such as name, location, size, and type—
for all files on that volume
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A typical file-system organization

11. 3. Directory Structure Cntd…
3.2 Operations Performed on Directory
• Search for a file - able to search a directory structure to find the entry for a
particular file
• Create a file - New files to be created and added to the directory
• Delete a file – remove it from the directory
• List a directory - able to list the files in a directory and the contents of the
directory entry for each file
• Rename a file - change the name when the contents or use of the file
changes
• Traverse the file system - access every directory and every file within a
directory structure
3.3 Single-Level Directory
Naming and Grouping problem Since all files are in the same directory
Loganathan R, CSE, HKBKCE 11

12. 3. Directory Structure Cntd…
3.4 Two-Level Directory
• Separate directory for each user user file directory (UFD)
• Master file directory (MFD) – All UFD entries
 Path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability
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13. 3. Directory Structure Cntd…
3.5 Tree-Structured Directories
Efficient searching , Grouping Capability
Current directory (working directory)
cd /spell/mail/prog
type list
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14. 3. Directory Structure Cntd…
• Efficient searching and Grouping Capability
• Current directory (working directory)
cd /spell/mail/prog
type list
• Absolute path begins at the root and follows a path down to the specified file
• Relative path defines path from the current directory
• Creating a new file is done in current directory
• Delete a file
rm <file-name>
• Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
mail
prog copy prt exp count
Deleting “mail”  deleting the entire subtree rooted by “mail”
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15. 3. Directory Structure Cntd…
3.6 Acyclic-Graph Directories
• Have shared subdirectories and files i.e. changes made by one is immediately
visible to the other
• New directory entry type
Link – another name (pointer)
to an existing file
Resolve the link – follow
pointer to locate the file
• Deletion of a link need
not affect the original file
• File entry is deleted, the
space for the file is
deallocated, leaving the
links dangling
• Preserve the file until all
references to it are
deleted.
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16. 3. Directory Structure Cntd…
3.7 General Graph Directory
How do we guarantee no cycles?
Allow only links to file not subdirectories
Garbage collection
Every time a new link is added use a cycle detection algorithm to determine
whether it is OK
16
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17. 4.File System Mounting
• A file system must be mounted before it can be accessed
• A unmounted file system (b) is mounted at a mount point
Mount point.
Existing system Unmounted volume
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18. 5. File Sharing
• Sharing of files on multi-user systems is desirable
• Sharing may be done through a protection scheme
• On distributed systems, files may be shared across a
network
• Network File System (NFS) is a common distributed
file-sharing method
5.1 Multiple Users
• User IDs (Owner)identify users, allowing permissions
and protections to be per-user
• Group IDs allow users to be in groups, permitting
group access rights
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19. 5. File Sharing
5.2 Remote File Systems
• Uses networking to allow file system access between systems
– Manually via programs like FTP
– Automatically, seamlessly using distributed file systems
– Semi automatically via the world wide web
• Client-server model allows clients to mount remote file systems from servers
– Server can serve multiple clients
– Client and user-on-client identification is insecure or complicated
• NFS is standard UNIX client-server file sharing protocol allow many-to-many
relationships.
• Distributed Information Systems (distributed naming services) such as LDAP(Light-
weight Directory Access Protocol), DNS(Domain Name System), NIS(Network
Information Service), Active Directory implement unified access to information
needed for remote computing
• Failure Modes
– Remote file systems add new failure modes, due to network failure, server failure
– Recovery from failure can involve state information about status of each remote
request
– Stateless protocols such as NFS include all information in each request, allowing easy
recovery but less security
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20. File Sharing – Consistency Semantics
• Consistency semantics specify how multiple users are
to access a shared file simultaneously
– Similar to process synchronization algorithms
• Tend to be less complex due to disk I/O and network latency (for
remote file systems
– Andrew File System (AFS) implemented complex remote file
sharing semantics
– Unix file system (UFS) implements:
• Writes to an open file visible immediately to other users of the same
open file
• Sharing file pointer to allow multiple users to read and write
concurrently
– AFS has session semantics
• Writes only visible to sessions starting after the file is closed
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21. Protection
• File owner/creator should be able to control:
– what can be done
– by whom
• Types of access
– Read
– Write
– Execute
– Append
– Delete
– List
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22. Access Lists and Groups
• Mode of access: read, write, execute
• Three classes of users
RWX
a) owner access 7  111
RWX
b) group access 6  110
RWX
c) public access 1  001
• Ask manager to create a group (unique name), say G, and add some
users to the group.
• For a particular file (say game) or subdirectory, define an appropriate
access.
owner group public
chmod 761 game
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23. Windows XP Access-control List Management
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