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Linux file systems

Linux file systems. Name: Peijun Li Student 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 .

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Linux file systems

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  1. Linux file systems Name: Peijun Li Student ID: 0100276 Prof. Morteza Anvari

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

  3. History of Linux file systemsMinux 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

  4. History of Linux file systemsMinux 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.

  5. History of Linux file systemsMinux 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.

  6. History of Linux file systemsMinux 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

  7. History of Linux file systemsMinux 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.

  8. Summery of features of different file system

  9. Basic File System Concept • Inodes • Directories • Links • Devices

  10. 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

  11. Inode structure :

  12. 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.

  13. Directories Inode table directory

  14. 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.

  15. 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.

  16. 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

  17. User process System call System call interface Linux kernel VFS Ext2 FS (etc.) Buffer Cache Device drivers I/O request Hardware Disk controller

  18. 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

  19. 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.

  20. 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.

  21. The structure of a block group Contains a redundant copy of crucial filesystem control informations and also contains a part of filesystem.

  22. Variable length entry In Ext2fs, directories are managed as linked lists of variable length entries. Each entry contains : inode number, entry length, file name and file length

  23. 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.

  24. 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

  25. 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.

  26. 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

  27. Results of Bonnie benchmark

  28. 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

  29. Thank you!

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