Linux file systems

Name: Peijun LiStudent ID: 0100276

Prof. Morteza Anvari

Introduction to Linux Unix-like operating system Free for everyone to use Multi-user Multi-tasking Internet ready Robust Multi platform Symmetrical Multi

Processors

History of Linux file systems

Minux FS--VFS--Ext FS--Xia FS--Ext 2 FS

In the early days, Linux was developed under the Minix operating system

Linus Torvalds implemented support for the Minux filesystem in Linux.

Limitations: block addresses are stored in 16 bits integers. maximal filesystem size is 64 MB maximal file name is 14 characters

History of Linux file systems

Minux FS--VFS--Ext FS--Xia FS--Ext 2 FS

In order to ease the addition of new filesystem into the Linux kernal, a Virtual File System layer(VFS) was developed and integrated into the Linux kernel.

History of Linux file systems

Minux FS--VFS--Ext FS--Xia FS--Ext 2 FS

In April 1992, a new filesystem called “Extended File System” was implemented and added to Linux 0.96c

Maximal size is 2 GB and maximal file name size is 255 characters

Limitations: No support for the separate access, inode modification, and data modification timestamps. Performance is too bad.

History of Linux file systems

Minux FS--VFS--Ext FS--Xia FS--Ext 2 FS

Released in Jan 1993 Heavily based on the Minux FS Kernel

code. Very stable. Provide long file names, support for bigger partitions and the three timestamps.

Limitation: not extensible

History of Linux file systems

Minux FS--VFS--Ext FS--Xia FS--Ext 2 FS Released in Jan, 1993 Based on Ext FS code with many

improvements. Was not stable. Right now is very

stable with the improvements and integrated new features.

Summery of features of different file system

Minix FS

Ext FS

Ext2FS

Xia FS

Max FS size 64 MB 2GB 4TB 2GBMax file size 64MB 2GB 2GB 64MBMax file name 16/30 c 255c 255c 248c3 times support NO NO YES YESExtensible NO NO YES NOVar. block size NO NO YES NOMaintained YES NO YES ?

Basic File System Concept

Inodes Directories Links Devices

Basic FS concept--Inodes

Each file is represented by an Inode Each inode contains the description of

the file: file type, access rights, owners, timestamps, size, pointers to data blocks

The addresses of data blocks allocated to a file are stored in its inode

:

                                                                

    

Inode structure

Basic FS concept--Directories

Directories are structured in a hierarchical tree. Each directory can contain files and subdirectories

A directory is a file containing a list of entries. Each entry contains an inode number and a file name

When a process uses a pathname, the kernel code searches in the directories to find the corresponding inode number, the Inode is loaded into memory for requests.

DirectoriesInode table

i1 name1

i2 name2

i1 name3

i4 name4

directory

Basic FS concept--links

Hard link can only be used to create a single file system;

can only point on files Symbolic link Does not point to an inode; It is possible to create cross-file systems

symbolic links Point to any type of file, even on nonexistent files.

Basic FS concept--Device special files

Character special files I/o operations in character mode Block special files requires data to be written in block mode When an I/O request is made on a special

file, it is forwarded to a (pseudo) device driver. A special file is referenced by a major number, which identifies the device type, and a minor number, which identifies the unit.

Virtual File System Is used during system calls acting on files The VFS is an indirection layer which

handles the file oriented system calls and calls the necessary functions in the physical file system code to do the I/O.

Knows about filesystem types supported in the kernel. It uses a table defined during the kernel configuration. Each entry in this table describes a filesystem type

User process

System call interface

VFS Ext2 FS (etc.)

Buffer Cache

Device drivers

Disk controller

System call

Linux kernel

HardwareI/O request

VFS structure A mounted filesystem descriptor contains

information common to filesystem types, pointers for functions provided by physical filesystem kernel code, and private data maintained by the physical filesystem code

An inode descriptor contains pointers to functions that can be used to

act on any file An open file descriptor contains pointer to functions which can only act on

open files

The Second Extended File System(Ext2fs)

supports standard Unix file types: regular files, directories, device special files and symbolic links

allows the users to modify the kernel behavior Can set attributes on a file or on a directory

Can select System V or BSD semantics at mount time. Allows the administrator to choose the logical block

size(1024, 2048, 4096 bytes). Implements fast symbolic links and keeps track of file

system state.

Physical structure A file system is made up of block

groups. Block groups are not tied to the physical layout of the blocks on the disk.

Boot selector

Block Group 1

Block Group2

……

Block Group N

The structure of a block group Contains a redundant copy of crucial

filesystem control informations and also contains a part of filesystem.

Super Block

FS descriptors

Block Bitmap

Inode Btmap

Inode Table

Data Blocks

Variable length entryIn Ext2fs, directories are managed as linked

lists of variable length entries. Each entry contains : inode number, entry length, file name and file length

Inode number

Entry length

Name length

File name

i1 16 05 File1

i2 40 09 Long_name

i3 12 02 f2

Performance optimization Use buffer cache management by performing

readaheads: when a block has to be read, the kernel code requests the I/O on several contiguous blocks.

Block groups are used to cluster together related inodes and data. This will reduce the disk head seeks made when the kernel reads an inode and its data blocks.

When writing data to a file, Ext2fs preallocates up to 8 adjacent blocks when allocating a new block. This will speed up future sequential read.

The Ext2fs library Provides routines which can be used to examine

and modify the data of an Ext2 filesystem. Many of the Ext2 utilities use the Ext2fs library.

This greatly simplifies the maintainance of these utilities.

Since the interfaces of the Ext2 library are abstract and general, new programs can be very easily written to access the Ext2fs

Provides access to several class of operations: filesystem, directories, inodes

The Ext2fs tools The mke2fs program: to initialize a partition to

contain an empty Ext2 File system The tune2fs program: to modify the filesystem

parameters The E2fsck program: to repair filesystem

inconsistencies after an unclean shutdown of the system. It is designed to run as quickly as possible.

The Debugfs program: to examine and change the state of a filesystem. Read-only by default.

Performance measurement Use Bonnie benchmark to measure

filesystem performance(Remy Card 1993): Bonnie Benchmark have been made on a

middle-end PC, based on a i486DX2 processor, using 16MB of memory and two 420MB IDE disks. The tests were run on Ext2 fs , Xia fs and on the BSD fast filesystem in asynchronous and synchronous mode

Results of Bonnie benchmark

Char Write(KB/s)

Block Write(KB/s)

Rewite (KB/s)

Char Read (KB/s)

Block Read (KB/s)

BSD Async 710 684 401 721 888BSD sync 699 677 400 710 878Ext2 fs 452 1237 536 397 1033Xia fs 440 704 380 366 895

Reference [Williaim Stallings 1998] Operating Systems

3rd edition 1998 [David A. Rustuling 1999] The Linux kernel

1999 [Remy Card 1993] Design and

Implementation of the second Extended Filesystem 1993

http://e2fsprogs.sourceforge.net/ext2.html

Thank you!