ch11 file system interface
TRANSCRIPT
Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9th Edition
Chapter 11: File-System Interface
11.2 Silberschatz, Galvin and Gagne ©2013Operating 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 fileFileChannel ch = raf.getChannel();// this locks the first half of the file - exclusiveexclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);/** Now modify the data . . . */// release the lockexclusiveLock.release();
11.11 Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9th Edition
File Locking Example – Java API (Cont.)
// this locks the second half of the file - sharedsharedLock = ch.lock(raf.length()/2+1, raf.length(),
SHARED);/** Now read the data . . . */// release the locksharedLock.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 nextwrite next resetno read after last write(rewrite)
Direct Access – file is fixed length logical recordsread nwrite nposition to nread nextwrite next rewrite nn = 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 2F 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-userGroup 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
RWXa) owner access 7 1 1 1
RWXb) group access 6 1 1 0
RWXc) 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 Gagne ©2013Operating System Concepts – 9th Edition
End of Chapter 11