Silberschatz, Galvin and Gagne 2002
11.1
Operating System Concepts
Chapter 11: File-System Interface
Chapter Outline
■File Concept
■Access Methods
■Directory Structure
■File System Mounting
■File Sharing
■Protection
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File System Concepts in Operating Systems, Thesis of Operating Systems
Various aspects of file systems in operating systems, including file concepts, access methods, directory structure, file sharing, protection, file systems, file types, file structure, access methods, directory structure, and file operations. It also discusses the unix open() and winnt/2000 createfile() functions, file types and extensions, file structure, sequential and direct access files, index and relative files, file system mounting, file sharing, protection, access lists and groups, and directory design goals.
Typology: Thesis
2017/2018
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Silberschatz, Galvin and Gagne
Chapter 11: File-System Interface
Chapter Outline
File Concept
Access Methods
Directory Structure
File System Mounting
File Sharing
Protection
Silberschatz, Galvin and Gagne
File SystemsFile SystemsFile SystemsFile Systems
File System consists of
✦
A collection of files
✦
A directory structure
✦
(possibly) partitions
Important Issues
✦
File protection
✦
The semantics of file sharing
Note:Note:Note:Note: Historically, operating systems and filesystems have been viewed as distinct entities.
From the perspective of the modern user, thisdistinction is often blurred.
Silberschatz, Galvin and Gagne
2002
File Attributes
Name
- only information kept in human-readable form.
Identifier – a unique tag (i.e., an internal number)that identifies the file within the file system.
Type
- needed for systems that support different types.
Location
- a 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.
Silberschatz, Galvin and Gagne
File Operations
Create
Write
Read
Reposition within file – file seek
Delete
Truncate
Open(
F
i
) – search the directory structure on disk for entry
F
i
, and move the content of entry to memory.
Close (
F
i
) – move the content of entry
F
i
in memory to
directory structure on disk.
Silberschatz, Galvin and Gagne
2002
Claypool Example:
Unix
open()
Under the Hood
int fid = open(“blah”, flags);read(fid, …);
User SpaceSystem Space
stdinstdoutstderr ...
File Structure
File
Descriptor
(whereblocks are)
(attributes)
(index)
(Per process)
(Per device)
Silberschatz, Galvin and Gagne
Claypool Example
: WinNT/
CreateFile()
Returns file object handle:HANDLE
CreateFile
lpFileName
// name of file
dwDesiredAccess
// read-write
dwShareMode
// shared or not
lpSecurity
// permissions
File objects used for all: files, directories, diskdrives, ports, pipes, sockets and console
Silberschatz, Galvin and Gagne
File StructureFile StructureFile StructureFile Structure
File types may be used to indicate the internalstructure of a file.
An OS may require
require
require require a file to have a specific structure
so that the OS will provide special operations for thosefiles conforming to the set of system-supported filestructures.
✦
e.g., VMS supported three defined file structures.
✦
Others (UNIX, MS-DOS) support a minimal
minimalminimalminimal number of file
structures.
This is an obvious tradeoff between flexibility andsystem support!
Silberschatz, Galvin and Gagne
Access Methods
Access methods determine the way that files areaccessed and read into memory.
Some systems only
only
only only support one access method while
other OS’s support many access methods.
Sequential Access
✦
The most common method used by editors andcompilers.
✦
Information is processed in order.
read nextwrite nextreset no
read after last write
(rewrite)
Silberschatz, Galvin and Gagne
Direct Access File
File is made up of fixed-length logical records thatallow programs to read and write records in noparticular order.
The files is viewed as a numbered sequence ofblocks or records.
Very useful in databases.
Direct Access
n
= relative block number}
read nwrite nposition to n
read nextwrite next
rewrite n
Silberschatz, Galvin and Gagne
Simulation of Sequential Access on a Direct-access File
Silberschatz, Galvin and Gagne
Directory StructureDirectory StructureDirectory StructureDirectory Structure
Partitions (or Volumes) – can be viewed as theabstraction of virtual disks.
virtual disks.virtual disks.virtual disks.
Disks can be partitioned into separate areas such thateach partition is treated as a separate storage device.
The other way --
a partition may be defined to be more
moremoremore
than onethan onethan onethan one disk device.
Partitions can store multiple
multiple
multiple multiple operating systems such that
a system can boot more than one OS.
Each partition contains information about files in adevice directorydevice directorydevice directorydevice directory (or a VTOC
VTOCVTOCVTOC – Volume Table of Contents).
Each directory records file attribute information.
Silberschatz, Galvin and Gagne
Directory Structure
A collection of nodes containing information about allfiles.
F 1
F 2
F 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.
Silberschatz, Galvin and Gagne
Information in a Device Directory
Name
Type
Address
Current length
Maximum length
Date last accessed (for archival)
Date last updated (for dump)
Owner ID (who pays)
Protection information (discuss later)
Silberschatz, Galvin and Gagne
Directory Operations
Search for a file
- need to find a particular entry or
be able to find file names based on a pattern match.
Create a file -
and add
add
add add its entry to the directory.
Delete a file
- and remove
remove
remove remove it from the directory.
List a directory –
list both the files in the directory
and the directory contents for each file.
Rename a file –
renaming may imply changing the
position of the file entry in the directory structure.
Traverse the file system –
the directory needs a logical
structure such that every directory and every filewithin each directory can be accessing efficiently.
efficiently.efficiently.efficiently.