523354 OPERATING SYSTEMS File Management •

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File Systems File Attributes File Operations File Types File Structure Access Methods Directory Structure File Sharing Protection File-System Implementation Allocation Methods

File Systems 

A file is

Contiguous logical address space  Managed by 

◦ OS ◦ Program

Logical address is the address generated by the CPU  exchange info  Physical address is the actual address of the process in the memory. 

File Systems 

Files represent ◦ programs  Source  object forms ◦ Data files  numeric, alphabetic, alphanumeric, or binary ◦ Free form  text files  Formatted file

Many different types of information may be stored in a file 1. text file 

1.

source file ◦ is a sequence of subroutines and functions, each of which is further organized as declarations followed by executable statements

2.

object file

3.

executable file ◦ is a series of code sections that the loader can bring into memory and execute.

File Attributes Name – only information kept in human-readable form  Identifier – unique tag (number) identifies file within file system  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 

File Operations 

Create ◦ Step1 : ◦ Step2 :

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Write

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Read ◦ Use a system call that specifies the name of the file and where (in memory) the next block of the file should be put.

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Reposition within file – file seek ◦ Move pointer to new location

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Delete ◦ Release all file space

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Truncate ◦ Erase the contents of a file but keep its attributes

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Open(Fi)

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Close (Fi)

Example: Unix open() int open(char *path, int flags [, int mode]) path is name of file  flags is bitmap to set switch 

◦ O_RDONLY, O_WRONLY… ◦ O_CREATE then use mode for perms 

On success, returns index



Several pieces of information are associated with an open file ◦ File pointer:  pointer to last read/write location

◦ 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:  Identify a disk location when a modification occurred with a file  cache of data access information

◦ Access rights:  per-process access mode information  OS decides

File Types – Name, Extension 

Filename.extension

File Structure



An OS ◦ Have to understand a file structure for processing ◦ supported file structures.  Executable File – run program

Access Methods  

Access methods determine the way that files are accessed and read into memory. Sequential Access

Access Methods 

Direct Access





Sequential Access read next write next reset no read after last write (rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number

Simulation of Sequential Access on Direct-access File

Access Methods 

Index and Relative Files

Directory Structure 

A collection of nodes containing information about all files.

Directory

Files

F1

F2

F3

F4 Fn

Both the directory structure and the files reside on disk. Backups of these two structures are kept on tapes.

Disk Structure 

Partitions (or Volumes) – can be viewed as the abstraction of virtual disks.



Partitions can store multiple operating systems such that a system can boot more than one OS

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Each partition contains information about files in a device directory, called directory

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Each directory records file attribute information.

A Typical File-system Organization

Directory Design Goal To organize the logical structure to obtain:  



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, …)

Directory Structure 

Single-Level Directory ◦ The simplest solution

◦ Advantages  Easy to support and understand. ◦ Disadvantages  The naming problem  The grouping problem

Directory Structure 

Single-Level Directory



Two-Level Directory

◦ Advantages  Solves the name-collision problem.  Isolates users from one another  a form of protection.  Efficient searching. ◦ Disadvantages  Restricts user cooperation.  No logical grouping capability (other than by user).



Two-Level Directory

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Tree-Structured Directories

◦ Advantages  Efficient searching  Grouping Capability ◦ Each process has a current directory (working directory)

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Tree-Structured Directories

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Tree-Structured Directories ◦ Absolute or relative path name ◦ Creating a new file is done in current directory. ◦ Delete a file rm ◦ Creating a new subdirectory is done in current directory. mkdir

Example: if in current directory /mail mkdir count mail prog

copy

prt exp count

Deleting “mail”  deleting the entire subtree rooted by “mail”.

relative path name

Absolute path name

Acyclic-Graph Directories

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New directory entry type ◦ Link – another name (pointer) to an existing file

Acyclic-Graph Directories

General Graph Directory 

When links are added to an existing treestructured directory, a general graph structure can be created.

General Graph Directory

File Sharing 

Sharing of files on multi-user systems is desirable



Sharing may be done through a protection scheme

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On distributed systems, files may be shared across a network

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Network File System (NFS) is a common distributed file-sharing method

File Sharing 1.

Multiple Users ◦ User IDs identify users, allowing permissions and protections to be per-user ◦ Group IDs allow users to be in groups, permitting group access rights

2.

Remote File Systems  Files are shared on network  FTP, world wide web, distributed file systems  Client-server model allows clients to mount remote file systems from servers  NFS is standard UNIX client-server file sharing protocol  CIFS is standard Windows protocol

Protection 

File owner/creator should be able to control: ◦ what can be done ◦ by whom

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Types of access ◦ ◦ ◦ ◦ ◦ ◦

Read Write Execute Append Delete List

Access Lists and Groups 

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

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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

chmod

Attach a group to a file chgrp

group 761

G

public

game

game

Windows Access control List Management

A Sample UNIX Directory Listing Drwxr-xr-x DirectoryOwnerGroupOtheruser

File-System Implementation     

File system resides on secondary storage (disks) File control block – storage structure consisting of information about a file Device driver controls the physical device Boot control block contains info needed by system to boot OS from that volume Per-file File Control Block (FCB) contains many details about the file

A Typical File Control Block

Allocation Methods 

An allocation method refers to how disk blocks are allocated for files:

1.

Contiguous allocation

2.

Linked allocation

3.

Indexed allocation

Contiguous Allocation 

Advantage

◦ Simple – only starting location (block #) and length (number of blocks) are required ◦ Provide a high efficiency for searching data

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Disadvantage

◦ Wasteful of space (dynamic storage-allocation problem) ◦ Files cannot grow

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CD (read only)

Linked Allocation

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Advantage ◦ Simple – need only starting address ◦ Free-space management system – no waste of space

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Disadvantage ◦ Spend more time for searching data ◦ Waste space to pointers block =

pointer



File-allocation table (FAT) ◦ disk-space allocation

Indexed Allocation I-nodes  bring all the pointers together into one location 

◦ index block.



Inside i-nodes, there are 4 block types 1. 2. 3. 4.

Direct blocks Single direct blocks Double direct blocks Triple direct blocks

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File size <= 10 Kb ◦ Used only direct blocks 10 blocks ◦ 10 blocks : each blocks has 1 kb

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File size > 10 kb and <= 266 kb ◦ Used direct blocks 10 blocks and Single direct blocks 1 block

◦ ◦ ◦ ◦

Single direct blocks = 1kb(1024 bytes) Links used per 4 bytes 1024/4 = 256 links 256 links can pointer to another 1 Kb block 10 kb + 256 kb = 266 kb

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File size > 266 and <=65802 kb ◦ Used direct blocks 10 blocks, 1 Single direct block and 1 double direct block ◦ Double direct block : 256*256 blocks

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File size > 65802 kb ◦ Used 1 Triple direct blocks

◦ Triple direct block : 256*256*256 blocks