Ch11 file system interface

Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
Chapter 11:
File-System Interface

11.2 Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
Chapter 11: File-System Interface
File Concept
Access Methods
Disk and Directory Structure
File-System Mounting
File Sharing
Protection

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Objectives
To explain the function of file systems
To describe the interfaces to file systems
To discuss file-system design tradeoffs, including access
methods, file sharing, file locking, and directory structures
To explore file-system protection

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File Concept
Contiguous logical address space
Types:
Data
 numeric
 character
 binary
Program
Contents defined by file’s creator
Many types
 Consider text file, source file, executable file

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File Attributes
Name – It is the only information which is in human-readable form.
Identifier – The file is identified by a unique tag(number) within file
system.
Type – It is needed for systems that support different types of files.
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
Many variations, including extended file attributes such as file
checksum
Information kept in the directory structure

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File info Window on Mac OS X

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File Operations
File is an abstract data type
Create
Write – at write pointer location
Read – at read pointer location
Reposition within file - seek
Delete
Truncate
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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Open Files
Several pieces of data are needed to manage open files:
Open-file table: tracks 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

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Open File Locking
Provided by some operating systems and file systems
Similar to reader-writer locks
Shared lock similar to reader lock – several processes can
acquire concurrently
Exclusive lock similar to writer lock
Mediates access to a file
Mandatory or advisory:
Mandatory – access is denied depending on locks held and
requested
Advisory – processes can find status of locks and decide
what to do

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File Locking Example – Java API
import java.io.*;
import java.nio.channels.*;
public class LockingExample {
public static final boolean EXCLUSIVE = false;
public static final boolean SHARED = true;
public static void main(String arsg[]) throws IOException {
FileLock sharedLock = null;
FileLock exclusiveLock = null;
try {
RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");
// get the channel for the file
FileChannel ch = raf.getChannel();
// this locks the first half of the file - exclusive
exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);
/** Now modify the data . . . */
// release the lock
exclusiveLock.release();

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File Locking Example – Java API (Cont.)
// this locks the second half of the file - shared
sharedLock = ch.lock(raf.length()/2+1, raf.length(),
SHARED);
/** Now read the data . . . */
// release the lock
sharedLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}finally {
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null)
sharedLock.release();
}
}
}

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File Types – Name, Extension

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File Structure
None - sequence of words, bytes
Simple record structure
Lines
Fixed length
Variable length
Complex Structures
Formatted document
Relocatable load file
Can simulate last two with first method by inserting
appropriate control characters
Who decides:
Operating system
Program

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Sequential-access File
•Data is accessed one record right after another is an order.
•Read command cause a pointer to be moved ahead by one.
•Write command allocate space for the record and move the
pointer to the new End Of File.
•Such a method is reasonable for tape.
•Example: Compilers usually access files in this way.

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Sequential-access File

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Direct Access-File
•This method is useful for disks.
•Random access file organization provides, accessing the records directly.
•Each record has its own address on the file. By the help of address it can
be directly accessed for reading or writing.
•The file is viewed as a numbered sequence of blocks or records.
•The records need not be in any sequence within the file and they need
not be in adjacent locations on the storage medium.
•User now says "read n" rather than "read next".
•"n" is a number relative to the address of file, not relative to an absolute physical
disk location.

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Indexed Sequential Access
•This mechanism is built up on base of sequential access.
•An index is created for each file which contains pointers to various
blocks.
•Index is searched sequentially and its pointer is used to access the file
directly.

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Access Methods
Sequential Access
read next
write next
reset
no read after last write
(rewrite)
Direct Access – file is fixed length logical records
read n
write n
position to n
read next
write next
rewrite n
n = relative block number
Relative block numbers allow OS to decide where file should be placed
See allocation problem in Ch 12

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Simulation of Sequential Access on Direct-access File

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Other Access Methods
Can be built on top of base methods
General involve creation of an index for the file
Keep index in memory for fast determination of location of
data to be operated on (consider UPC code plus record of
data about that item)
If too large, index (in memory) of the index (on disk)
IBM indexed sequential-access method (ISAM)
Small master index, points to disk blocks of secondary
index
File kept sorted on a defined key
All done by the OS
VMS operating system provides index and relative files as
another example (see next slide)

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Example of Index and Relative Files

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Directory Structure
A collection of nodes containing information about all files
F 1 F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk

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Disk Structure
Disk can be subdivided into partitions
Disks or partitions can be RAID protected against failure
Disk or partition can be used raw – without a file system, or
formatted with a file system
Partitions also known as minidisks, slices
Entity containing file system known as a volume
Each volume containing file system also tracks that file system’s
info in device directory or volume table of contents
As well as general-purpose file systems there are many
special-purpose file systems, frequently all within the same
operating system or computer

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A Typical File-system Organization

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Types of File Systems
We mostly talk of general-purpose file systems
But systems frequently have may file systems, some general- and
some special- purpose
Consider Solaris has
tmpfs – memory-based volatile FS for fast, temporary I/O
objfs – interface into kernel memory to get kernel symbols for
debugging
ctfs – contract file system for managing daemons
lofs – loopback file system allows one FS to be accessed in
place of another
procfs – kernel interface to process structures
ufs, zfs – general purpose file systems

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Operations Performed on Directory
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system

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Directory Organization
Efficiency – locating a file quickly
Naming – convenient to users
Two users can have same name for different files
The same file can have several different names
Grouping – logical grouping of files by properties, (e.g., all
Java programs, all games, …)
The directory is organized logically to obtain

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Single Level Directory
In Single Level Directory all files are in the same directory.
Advantage:
Easy to support and understand.
Disadvantage:
a) Since all files are in the same directory, they must have unique name.
b) Requires unique file names {the naming problem}.
c)If two user call their data free test, then the unique name rule is violated.
d) Files are limited in length.
e) Even a single user may find it difficult to remember the names of all
files as the number of file increases.

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Single-Level Directory
A single directory for all users
Naming problem
Grouping problem

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Each user has their own directory space.
File names only need to be unique within a given user's directory.
A master file directory is used to keep track of each users directory, and
must be maintained when users are added to or removed from the system.
When user refers to a particular file, only his own user file directory is
searched.
Thus different users may have files with same name. To have a particular
file uniquely, in a two level directory, we must give both the user name and
file name.
A two level directory can be a tree or an inverted tree of height 2
The root of a tree is Master File Directory (MFD).
Its direct descendents are User File Directory (UFD). The descendents of
UFD's are file themselves.
The files are the leaves of the tree.
Two-Level Directory

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Advantage of Two Level Directory
Isolates users from one another ! a form of protection.
Efficient searching
Solves the name-collision problem.
Disadvantage
Restricts user cooperation.
No grouping capability

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Two-Level Directory
Separate directory for each user
Path name
Can have the same file name for different user
Efficient searching
No grouping capability

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Tree-Structured Directories
This generalization allows user to create their own subdirectories and it
recognize their file accordingly.
The Ms-Dos system structured as a tree.
The tree has a root directory.
Every file has a unique path name.
A path name is the path from the root.
A directory contains set of files or subdirectories.
A directories have the same internal format
One bit in each directory entry defines the entry as a file or as a
subdirectory.
Each process has a current directory.
When a reference is made to a file, the current directory is searched.
Special calls are used to create and delete directories.

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 Advantage:
1.Allows user to create their own directory.
2.Supports double click execution functionality
3.Efficient searching
4.Grouping Capability
 Disadvantage:
1.Tree structure directory can be larger than the two level directory.
2.Subdirectories can not be share between users

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Tree-Structured Directories

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Tree-Structured Directories (Cont.)
Efficient searching
Grouping Capability
Current directory (working directory)
cd /spell/mail/prog
type list

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Tree-Structured Directories (Cont)
Absolute or relative path name
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
Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”

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Acyclic Graph Directory
Acyclic Graph is the graph with no cycles. It allows directories to
share sub directories and files. With a shared file, only one actual
file exists, so any changes made by one person are immediately
visible to the another.
Advantage:
1.More flexible than is a simple tree structure
2.Allow user to share their files
3. Allow multiple directories to contain same file
 Disadvantage:
1)Protection problem

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Acyclic-Graph Directories
Have shared subdirectories and files

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Acyclic-Graph Directories (Cont.)
Two different names (aliasing)
If dict deletes list ⇒ dangling pointer
Solutions:
Backpointers, so we can delete all pointers
Variable size records a problem
Backpointers using a daisy chain organization
Entry-hold-count solution
New directory entry type
Link – another name (pointer) to an existing file
Resolve the link – follow pointer to locate the file

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General Graph Directory
When links are added to an existing tree-structured directory, a general
graph structure can be created.
A general graph can have cycles.
Disadvantage:
problems when searching or traversing file system.

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General Graph Directory

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General Graph Directory (Cont.)
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

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File System Mounting
A file system must be mounted before it can be accessed
A unmounted file system (i.e., Fig. 11-11(b)) is mounted at a
mount point

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Mount Point

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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
If multi-user system
User IDs identify users, allowing permissions and
protections to be per-user
Group IDs allow users to be in groups, permitting group
access rights
Owner of a file / directory
Group of a file / directory

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File Sharing – 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
CIFS is standard Windows protocol
Standard operating system file calls are translated into remote calls
Distributed Information Systems (distributed naming services) such
as LDAP, DNS, NIS, Active Directory implement unified access to
information needed for remote computing

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File Sharing – Failure Modes
All file systems have failure modes
For example corruption of directory structures or other non-
user data, called metadata
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 v3 include all information in
each request, allowing easy recovery but less security

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File Sharing – Consistency Semantics
Specify how multiple users are to access a shared file
simultaneously
Similar to Ch 5 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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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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Access Lists and Groups
Mode of access: read, write, execute
Three classes of users on Unix / Linux
RWX
a) owner access 7 ⇒ 1 1 1
RWX
b) group access 6 ⇒ 1 1 0
RWX
c) public access 1 ⇒ 0 0 1
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.
Attach a group to a file
chgrp G game

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Windows 7 Access-Control List Management

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A Sample UNIX Directory Listing

Silberschatz, Galvin and GagneOperating System Concepts – 9th
Edition
End of Chapter 11

Ch11 file system interface

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Ch11 file system interface