Fall 2013 SILICON VALLEY UNIVERSITY CONFIDENTIAL 1 Introduction to Embedded Systems Dr. Jerry Shiao, Silicon Valley University

SILICON VALLEY UNIVERSITY CONFIDENTIAL 2 Pearson Addison-Wesley File System Review Inode a) Logical structure of current directory. File, Chapter3, is created in current directory. b) Contents of current directory. Directory entry contains pair. c) Relationship among a directory entry, Inode Table, and physical file contents. -Inode # index into the Inode Table. -Inode Table entry contains the physical location of file on disk.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 3 Spring 2014 File System  Virtual File System (VFS) User space has the applications and glibc (provides API file system calls: open, read, write, close). System call interface acts as a switch, funneling system calls from user space to the appropriate endpoints in kernel space. VFS exports APIs and abstracts them to individual file systems. Inode cache and Directory cache provides a pool of recently-used file system objects. Directory cache speeds up translation from file pathname to inode. Individual file systems exports APIs that is used by the VFS. Buffer cache buffers the requests between the file systems and the block devices that they manipulate (for faster access).

SILICON VALLEY UNIVERSITY CONFIDENTIAL 4 Spring 2014 File System  Virtual File System (VFS) File Object: Stores info about interaction between open file and process. Dentry Object: Stores info about linking directory entry (file name) and file. Inode Object: Stores info about specific file (inode number identifies file). The file control block. Superblock Object: Stores info about mounted filesystem. The filesystem control block. Process 1 Process 2 File Object Dentry Object Dentry Object Dentry Cache Inode Object Superblock Object Inode Cache Process 1 Process 2 File Object Dentry Object Dentry Object Dentry Cache Inode Object Superblock Object Inode Cache Disk File

SILICON VALLEY UNIVERSITY CONFIDENTIAL 5 Spring 2014 Linux File System  Virtual File System (VFS) struct file_system_type links together currently- supported file systems (/proc/filesystems). struct vfsmount links together file systems currently mounted. Link to struct superblock. struct superblock represents a file system.  Manage inodes: alloc_inode, destroy_inode, read_inode, write_inode, sync_fs. struct inode represents an object (file or directory) in the file system with unique identifier. Metadata to manage objects in the file system  struct inode_operations: Operations that operate on inode.  struct file_operations: Methods related to files and directories (the standard system calls).

SILICON VALLEY UNIVERSITY CONFIDENTIAL 6 Spring 2014 Linux File System  Virtual File System (VFS) To achieve the abstraction (i.e. “black box operation) to the user, common API to the user through glibc library and common callback function signature to the I/O functions. Inode represents an object in the file system with a unique identifier (translating filename). struct file_operations abstractions (i.e. read/write/open ) allow all I/O operations to have common interface. The indirect calls (i.e. callback functions) are APIs specific to the file system.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 7 Pearson Addison-Wesley File System Hard Links  Hard link creates a file entry (new file name) in directory with the same inode index as the linked file.  ln [ options ] existing-file new-file  ln [ options ] existing-file-list directory  - f : Force creation of link.  - n : Do not create the link if “new-file” exists.  - s : Create symbolic link to “existing-file”.  NOTE: : Create a hard link to “existing-file”. : The user MUST have execute permission for all the directories in the path leading to the file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 8 Pearson Addison-Wesley File System Hard Links Current Directory  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 1 sau users :23 Chapter3  $ ln Chapter3 Chapter3.hard  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 2 sau users :23 Chapter rw-r--r-- 2 sau users :23 Chapter3.hard  $ rm Chapter3  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 1 sau users :23 Chapter3.hard  $ Create hard link. The inode index for linked file and file are the same. Link count is 2. Remove only removes the entry in the directory. File is physically still on disk. Link count is 1.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 9 Pearson Addison-Wesley File System Link Count  Number of different directory entries pointing to inode of the object (i.e. directory or file).  File: Number of hard links to that file. $ ls -il memo rw-r--r-x 1 sau users :16 memo6  Directory: Number of directory entries connecting local directory to Linux file system. $ ls -a... students students_2 students.gz $ ls -ld drwxr-xr-x 2 student1 student :06. $ mkdir test_dir $ ls -a... students students_2 students.gz test_dir $ ls -ld drwxr-xr-x 3 student1 student :06.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 10 Pearson Addison-Wesley File System Create Hard Link a) Logical structure of current directory. Hard link, Chapter3.hard, is linked to file, Chapter3 in current directory. b) Contents of current directory. Directory entry contains pair for Chapter3.hard. The inode# is same as file Chapter3. c) Relationship among a directory entry, Inode Table, and physical file contents. -Inode # index into the Inode Table for both Chapter3 and Chapter3.hard.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 11 Pearson Addison-Wesley File System Hard Link Different Directory

SILICON VALLEY UNIVERSITY CONFIDENTIAL 12 Pearson Addison-Wesley File System Hard Links Different Directory  $ ls -il memos/memo rw-r--r-x 1 sau users :16 memos/memo6  $ ln memos/memo6 memo6.hard  $ ls -il memos/memo rw-r--r-x 2 sau users :16 memos/memo6  $ ls -il memo6.hard rw-r--r-x 2 sau users :16 memo6.hard  $  $ rm memos/memo6  $ ls -il memos/memo6 ls: cannot access memos/memo6: No such file or directory  $ ls -il memo6.hard rw-r--r-x 1 sau users :16 memo6.hard  $ Create hard link. The inode index for linked file and file are the same. Link count is 2. Remove only removes the entry in the directory. File is physically still on disk. Link count is 1.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 13 Pearson Addison-Wesley File System Hard Links Limitations  Cannot use Hard Links between files on different file systems. # ln /usr/bin/zip my_zip ln: creating hard link `my_zip' => `/usr/bin/zip': Invalid cross-device link  Moving a hard linked file to different file system causes physical copy of file to be made. $ ls -il memo6.hard rw-r--r-x 2 sau users :25 memo6.hard $ ls -il memos/memo rw-r--r-x 2 sau users :25 memos/memo6 $ mv memo6.hard /tmp $ ls -il /tmp/memo6.hard rw-r--r-x 1 sau users :25 /tmp/memo6.hard

SILICON VALLEY UNIVERSITY CONFIDENTIAL 14 Pearson Addison-Wesley File System Soft Links  Does not have any of the Hard Link issues.  Soft link creates a new file entry (new file name and new inode index) in directory.  ln [ options ] existing-file new-file  ln [ options ] existing-file-list directory  - f : Force creation of link.  - n : Do not create the link if “new-file” exists.  - s : Required. Create symbolic link to “existing-file”.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 15 Pearson Addison-Wesley File System Soft Links Current Directory  $ ls -il Chapter rw-r--r-x 1 sau users :47 Chapter3  $ ln -s Chapter3 Chapter3.soft  $ ls -il Chapter3 Chapter3.soft rw-r--r-x 1 sau users :47 Chapter lrwxrwxrwx 1 sau users :12 Chapter3.soft -> Chapter3  $ Create soft link. 1)The inode index for linked file and file are different. These are different files. 2)Original file is file type “-”, an ordinary file. Link file is type “l”, a link file. 3)Link count 1. 4)File sizes are different. Link file is 8 bytes, the length of “Chapter3”, the pathname is placed in the link file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 16 Pearson Addison-Wesley File System Soft Links Create soft link. 1)The inode index for linked file and file are different. These are different files. 2)Original file is file type “-”, an ordinary file. Link file is type “l”, a link file. 3)Link count 1. 4)File sizes are different. Link file is 8 bytes, the length of “Chapter3”, the pathname is placed in the link file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 17 Pearson Addison-Wesley File System Soft Links Pros and Cons  Pros Establish symbolic link between files across file systems. Establish symbolic link between directories. Symbolic linked file edited by editor does not change filename.  Cons If the file that the symbolic link points to is moved from one directory to another, it can no longer be accessed via the link. UNIX has to support an additional file type (the link type) and a new file has to be created for every link. Symbolic links also result in slow file operations because, for every reference to the file, the link file has to be opened and read in order to reach the actual file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 18 Pearson Addison-Wesley Assignment 9

SILICON VALLEY UNIVERSITY CONFIDENTIAL 19 Pearson Addison-Wesley Assignment 9