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Module #11 – Linux Case Study

� Lecture #2

• Linux

� Readings: Silberschatz, 20.1-20.11

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Linux Overview

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Files and File Systems

� The file system is the most visible aspect of an operating system – it provides the resource abstractions typically associated with secondary storage.

� Files are central to most programs – almost all programs read input and produce output.

� Files exist independent of any program – a particular file can be accessed by more than one program.

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File Properties

� Long-term existence: stored on disk (or other secondary storage device) and information retained over time.

� Sharable: can be used by any process that has the appropriate access permissions.

� Structured: internal structure organized in a way that is convenient for the programs which use the file.

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File Management

� Many variants (evolution over time).

� Traditional UNIX:

• Tree-structured directory system

• Files viewed as a stream (sequence) of bytes

• File permissions and groups used to control access to files

• Inodes ("index nodes") used to manage files

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File Types

� Regular: stream of bytes

� Directory: contains list of files (names and inodenumbers) in the directory

� Device: maps file name to physical device (such as terminal or printer)

� Symbolic link: contains path of target file

� Socket: access to networking protocols

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Linux Virtual File System

� Flexible – supports any file management system and file structure.

� VFS provides interface between user's view of the Linux file system and details of a specific file system (EXT4, NFS, AFS, FAT, IBM's JFS, etc).

� Maps Linux system calls to routines which are "native" to the specific file system.

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Linux Virtual File System

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Linux Virtual File System

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Linux Virtual File System

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Linux Virtual File System

� VFS independent of any specific file system.

� System calls related to files mapped to functions within the specific file system.

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Secondary Storage Management

� A directory is a file (on disk) which contains pairs of file names and inode numbers.

� Copies of recently referenced directories are cached in memory (for fast access).

. 20333131

.. 9913609

Admin 20333126

example01 23353850

example01.c 23353851

example02 36472807

example02.c 36472808

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Secondary Storage Management

� An inode number is an index into the inode table (on disk, but not in a file).

� Each inode is a record (256 bytes).

� The inode table is created when the file system is initialized.

� Copies of recently referenced inodes are cached in memory (for fast access).

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Inodes

� Directory entry contains inodenumber and file name.

� Inode contains all other information about the file.

� Data blocks contain contents of file.

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Inode Structure

Fields within an inode:

• owner (user, group)

• permissions (rwx for owner, group, world)

• time stamps (last access, last write, last modification to inode)

• file type: regular, directory, pipe, etc

• file size (in bytes)

• info on blocks where data is stored

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Inode Structure

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Example

� Inode contains the meta information, as well as fifteen 8-byte block addresses:

• Twelve pointers to data blocks

• One "single indirect" pointer

• One "double indirect" pointer

• One "triple indirect" pointer

� Small files (12 or fewer data blocks) can be accessed directly; larger files require one or more levels of indirection.

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direct (11)

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Example

� Block size: 4 Kbytes

� Block addresses: 8 bytes wide

� Maximum size file: 512+ Gbytes

Level Number of Blocks Number of Bytes

Direct 12 48K

Single Indirect 512 2M

Double Indirect 512 x 512 = 256K 1G

Triple Indirect 512 x 256K = 128M 512G

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File Access Methods

� Linux approach (inodes, pointers to data blocks) supports both sequential access and direct access.

� Sequential access: move through the file, from first byte to last byte.

• location of first byte easily found

• system maps logical file position to offset in disk block

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File Access Methods

� Direct access: access arbitrary byte (move forward or backward in file).

• system maps logical file position to offset in disk block (current position)

• system calculates location of desired position

� System calls to support access methods

• fstat()

• lseek()

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File System Layout

Disk drive divided into partitions (each with an integer number of cylinders)

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File System Layout

� Boot block: bootstrap code

� Superblock: describes state of file system

� Inode map: inodes free or in use

� Block bitmap: blocks free or in use

� Inode table: array of all inodes

� Data blocks: sectors on disk (each block can belong to one file)

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Performance Issues

� Free list eventually becomes distributed randomly over the disk as files are created and destroyed; hence, files are widely distributed over disk.

� Files in same directory are not necessarily near one another.

� Inodes are not necessarily near their respective files.

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Disk Scheduling

� Earlier versions of Linux used a variant on LOOK (the Linux Elevator algorithm).

� More recent versions of Linux support multiple scheduling algorithms.

• CFQ (Completely Fair Queuing) Scheduling

• Deadline Scheduling

• NOOP scheduling

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Disk Scheduling

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Disk Scheduling

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Disk Scheduling

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Network Structure

� Key area of functionality for Linux

• Supports standard Internet protocols for UNIX to UNIX communications

• Supports protocols native to non-UNIX operating systems (such as Appletalk and IPX)

� Three layers in the Linux kernel:

• The socket interface

• Protocol drivers

• Network device drivers

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Network Structure

� Most important set of protocols is the Internet protocol suite.

• It implements routing between different hosts anywhere on the network

• On top of the routing protocol are built the UDP, TCP and ICMP protocols

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