ch11 file system

Course 11: File-System

11.2 Silberschatz, Galvin and Gagne ©2008Operating System Concepts

Course 10: Review

Virtual memory – separation of user

logical memory from physical memory.

Only part of the program needs to be

in memory for execution

Logical address space can therefore

be much larger than physical address

space

Allows address spaces to be shared

by several processes

Allows for more efficient process

creation


Course 10: Review – Demand Paging


Course 10: Review – Copy on Write


Course 10: Review – Page Replacement


Course 11: File-System

File Concept

Access Methods

Directory Structure

Protection

File-System Structure

File-System Implementation

Allocation Methods


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

To describe the details of implementing local file systems and

directory structures


File Concept


File Concept

Resource for storing information

Contiguous logical address space

User point of view

Interface to information (usually hierarchical)

OS point of view

Data structures and allocation management algorithms

Files contains

Data

Meta-data - FCB


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 – e.g. XML

Who decides:

Operating system

Program


File Types – Name, Extension


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

File is an abstract data type

Operations:

Create

Write

Read

Reposition within file

Delete

Truncate


Open Files

Several pieces of data are needed to manage 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


Access Methods


Sequential-access File


Access Methods

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 a Direct-access File


Example of Index and Relative Files


Directory Structure


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

Backups of these two structures are kept on tapes


A Typical File-system Organization


Operations Performed on Directory

Search for a file

Create a file

Delete a file

List a directory

Rename a file

Traverse the file system


Organize the Directory (Logically) 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, …)


Single-Level Directory

A single directory for all users

Naming problem

Grouping problem


Two-Level Directory

Separate directory for each user

Path name

Can have the same file name for different user

Efficient searching

No grouping capability


Tree-Structured Directories

Efficient searching

Grouping capability


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

mail

prog copy prt exp count

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


Acyclic-Graph Directories

Have shared subdirectories and files


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


Protection


Protection

File owner/creator should be able to control:

what can be done

by whom

Types of access

Read

Write

Execute

Append

Delete

List


Access Lists and Groups

Mode of access: read, write, execute

Three classes of users

RWX

a) owner access 7 1 1 1RWX

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.

owner group public

chmod 761 game

Attach a group to a file

chgrp G game


Windows XP Access-control List Management


A Sample UNIX Directory Listing


File System Structure


File-System Structure

File structure

Logical storage unit

Collection of related information

File system resides on secondary storage (disks)

File system organized into layers

File control block – storage structure consisting of information

about a file


Layered File System


File System Implementation


OS File Control Structures

A boot control block (per volume) - contain information needed by

the system to boot an operating system from that volume

A volume control block (per volume) contains volume (or

partition) details, such as the number of blocks in the partition, size

of the blocks, freeblock count and free-block pointers

A directory structure per file system is used to organize the files.

In UFS, this includes file names and associated inode numbers. In

NTFS it is stored in the master file table.

A per-file FCB contains many details about the file, including file

permissions,ownership, size, and location of the data blocks. In

UFS, this is called the inode.


A Typical File Control Block


In-Memory File System Structures

The following figure illustrates the necessary file system structures

provided by the operating systems.

Figure 12-3(a) refers to opening a file.

Figure 12-3(b) refers to reading a file.


In-Memory File System Structures


Virtual File Systems

Virtual File Systems (VFS) provide an object-oriented way of

implementing file systems.

VFS allows the same system call interface (the API) to be used for

different types of file systems.

The API is to the VFS interface, rather than any specific type of file

system.


Schematic View of Virtual File System


Directory Implementation

Linear list of file names with pointer to the data blocks.

simple to program

time-consuming to execute

Hash Table – linear list with hash data structure.

decreases directory search time

collisions – situations where two file names hash to the same

location

fixed size


Alocation Methods


Allocation Methods

An allocation method refers to how disk blocks are allocated for

files:

Contiguous allocation

Linked allocation

Indexed allocation


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

Wasteful of space (dynamic storage-allocation problem)

Files cannot grow


Contiguous Allocation of Disk Space

IBM 360 Pros: Fast Sequential

Access, Easy Random access

Cons: External Fragmentation/Hard to grow files Free holes get

smaller and smaller

Could compact space, but that would be reallyexpensive


Extent-Based Systems

Many newer file systems (I.e. Veritas File System) use a modified

contiguous allocation scheme

Extent-based file systems allocate disk blocks in extents

An extent is a contiguous block of disks

Extents are allocated for file allocation

A file consists of one or more extents.


Linked Allocation

Each file is a linked list of disk blocks: blocks may be scattered

anywhere on the disk.

pointerblock =


Linked Allocation (Cont.)

Simple – need only starting address

Free-space management system – no waste of space

No random access

File-allocation table (FAT) – disk-space allocation used by

MS-DOS and OS/2.


Linked Allocation

Pros: Can grow files

dynamically, Free list

same as file

Cons: Bad Sequential

Access (seek between

each block),

Unreliable

(lose block, lose rest

of file)

Serious Con: Bad

random access!!!!

Technique originally

from Alto (First PC,

built at Xerox)

No attempt to

allocate contiguous

blocks


File-Allocation Table

MSDOS links pages together to create a file Links not in pages, but

in the File Allocation Table (FAT) FAT contains an

entry for each block on the disk

FAT Entries corresponding to blocks of file linked together

Access properties: Sequential access

expensive unless FAT cached in memory

Random access expensive always, but reallyexpensive if FAT not cached in memory


Indexed Allocation

Brings all pointers together into the index block.

Logical view.

index table


Example of Indexed Allocation

System Allocates file

header block to hold

array of pointers big

enough to point to all

blocks

User pre-declares

max file size;

Pros: Can easily

grow up to space

allocated for index

Random

access is fast

Cons: Clumsy to grow

file bigger than table

size

Still lots of

seeks: blocks may be

spread over disk


Indexed Allocation (Cont.)

Need index table

Random access

Dynamic access without external fragmentation, but have overhead of index block.


Indexed Allocation – Mapping (Cont.)

outer-index

index table file


Combined Scheme: UNIX (4K bytes per block)

UNIX 4.1 Pros and cons

Pros:

Simple (more or less)

Files can easily expand (up to a point)

Small files particularly cheap and easy

Cons:

Lots of seeks

Very large files must read many indirect blocks (four I/Os per block

End of Course 11

ch11 file system

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