See this page as a slide show
The Filesystem
Original slides from Dr. James Walden at Northern Kentucky University.
Topics
- Overview
- Pathnames
- Mounting
- Structure
- Organization
- File types
- Kernel Data Structures
Overview
- Basic purpose of the Linux filesystem:
To represent and organize the system’s storage resources, i.e.,
directories and files.
- Because the filesystem was well organized and easy to access,
system programmers uses it to store other resources:
- Processes
- Devices
- Mount Points
- Kernel Data
- Communication Channels
Filesystem Components
- Namespace:
A way of naming things and organizing them in a hierarchy
- Application Programmer Interface:
A set of system calls for navigating and manipulating objects
- Security model:
A scheme for protecting, hiding, and sharing things
- Implementation:
Software that ties the logical model to the actual hardware
Filesystem Types
- ext2, ext3, ext4: Linux
- FAT16, FAT32, NTFS: Windows
- HFS, HFS+: Apple
- XFS: Silicon Graphics
- NFSv2, NFSv3, NFSv4: Sun (distributed)
- ISO 9660, UDF: Optical Discs
https://en.wikipedia.org/wiki/Comparison_of_file_systems
Hierarchy
Files are located by traversing a directory tree:
- Root directory:
Represented by single forward slash (
/)
- Home directory:
Represented by the tilde symbol (
~)
- Working directory:
Processes have a current working directory (
.)
- Parent directory:
All directories except root have parent (
..)
- Absolute path:
Pathname all the way from root (
/usr/local/bin)
- Relative path:
Pathname from current location (
../local/bin)
Linux filesystem is a single unified hierarchy, unlike Windows.
Filenames
- File, filename, path, pathname
- Files are represented by strings
- Pathname includes directory structure
- File and directory names
- Spaces and other special characters can be used
- Anything but slash or null is ok.
- Periods are followed by file extension, periods aren’t special
- Except that
ls, by default, ignores files starting with one.
- Filenames are limited to 255 characters (Linux)
- Pathnames are limited to 4095 characters (Linux)
- Linux
find command provides recursive listing
Mounted Filesystems
Global filesystem contains mounted filesystems:
- Textbook uses “file tree” for global filesystem
- Mounted filesystems are attached to file tree
- Mounted filesystems are usually disk partitions
- Also can be network file servers
- Each must obey the proper API
- Attached to the tree with the
mount command
Mounting
mount /dev/hd4 /MST3K
- Installs filesystem stored on disk under the path
/MST3K
- Files that were originally at
/MST3K no longer accessible!
- Filesystem navigation occurs seamlessly
cat /etc/fstab
- List of filesystems customarily mounted on system
- Filesystems can be checked using
fsck command
- Shows layout of system and allows
mount -a
- Show mounted filesystems and their type with
df -T
Unmounting
umount /MST3K
- Detaches a file system
- Cannot unmount a file system that is busy
- No open files
- No processes whose current working directory is in the file system
- No executable programs from the filesystem
umount -l /MST3K
- Lazy unmount, makes it unavailable
- Waits for none of the above to be true
- Then physically unmounts
Identify Processes
fuser -mv /MST3K
(Shows all open files, processes, and executables)
| Code | Meaning |
|---|
| f | Process has an open file |
| c | Process has a current working directory |
| e | Process is currently executing a file |
| r | Process has set a root directory (chroot) |
| m | Process has mapped a file or a shared library |
Filesystem Structure
- Blocks that store user data contained in files
- Blocks that stores metadata with structural info
- Superblock: filesystem type, size, status
- Objects in filesystem represented by inodes
- inode has file type, permissions, owner, group,
file size, number of links, access control,
file creation, access, modification times
- Directories are files with list of files
- Also stores the mapping from files to inodes
Examples
$ ls -l /etc/passwd
-rw-r--r--. 1 root root 2733 May 17 12:46 /etc/passwd
$ ls -i /etc/passwd
1468033 /etc/passwd
$ stat /etc/passwd
File: `/etc/passwd’
Size: 2733 Blocks: 8 IO Block: 4096 regular file
Device: 802h/2050d Inode: 1468033 Links: 1
Access: (0644/-rw-r--r--) Uid: (0/ root) Gid: (0/ root)
Access: 2012-08-29 18:50:01.213638229 -0600
Modify: 2012-05-17 12:46:25.000000000 -0600
Change: 2012-05-18 18:41:06.000000000 -0600
Filesystem Organization
- Organization has always been haphazard
- Several naming schemes used, some incompatible
- That’s what you get when things evolve.
Standard directories
| Directory | Meaning | Directory | Meaning |
|---|
/boot | Boot directory | /usr | Most standard programs |
/dev | Device files | /var | Spool directories |
/etc | Critical system files | /home | Mount point for users |
/sbin | System utilities | /lib | Libs and parts of the C compiler |
/bin | Important utilities | /media | Removable media |
/tmp | Temp files | /opt | Optional applications |
Standard directories
/usr/bin | Most commands and executables |
/usr/include | Header files |
/usr/lib | Libraries, support files for standard programs |
/usr/local | Local software |
/usr/local/bin | Local executables |
/usr/local/… | Other local (etc, lib, sbin, src) |
Standard directories
/usr/man | Man pages |
/usr/sbin | Less essential sysadmin commands |
/usr/share | Common to multiple systems |
/usr/share/man | Shared man pages |
/usr/src | Source code for nonlocal packages |
Standard directories
/var/adm | Logs, system setup records |
/var/log | System log files |
/var/spool | Spooling directories (mail, printers) |
/var/tmp | More temp space (preserved between boots) |
File Types
File types in the Linux filesystem:
- Regular files (-)
- Directories (d)
- Character devices (c)
- Block devices (b)
- Local domain sockets (s)
- Named pipes (p)
- Symbolic links (l)
File types
$ ls -ld
crw------- 1 root root 5, 1 Aug 24 15:24 /dev/console
brw-rw---- 1 root disk 8, 0 Aug 24 15:23 /dev/sda
srw-rw-rw- 1 root root 0 Aug 24 15:23 /dev/log
File type encoding
| File type | Symbol | Created by | Removed by |
|---|
| Regular file | - | cp, mv, vi | rm |
| Directory | d | mkdir, cp -r | rmdir, rm -r |
| Character device file | c | mknod | rm |
| Block device file | b | mknod | rm |
| Local domain socket | s | socket(2) | rm |
| Named pipe | p | mknod | rm |
| Symbolic link | l | ln -s | rm |
Regular file
- File of bytes
- Linux imposes no structure
- Sequential and random access allowed
Directories
- Contains named references to other files
. Refers to current directory
.. Refers to parent directory
- File’s name stored within the parent’s directory
- Not in file itself. Really!
- More than one directory can refer to a file (links)
ln creates hard links, removed with rm
- Indistinguishable from first entry
Character and Block Device files
- Device files allow programs to communicate with
hardware and peripherals
- Modules that know how to talk to the devices are
linked into the kernel when built
- Device file not the same as a device driver
- File is just a rendezvous point
- Device files characterized by major, minor numbers
- Major number tells kernel which driver
- Minor number tells driver which device
Local domain sockets
- Sockets are communication mechanisms
- Allow communication between processes
- Or more importantly, sockets control network communication
- Local domain sockets
- Only accessible from the local host
- Filesystem object rather than a network port
- Visible to other processes
- Accessible only by connected processes
- Examples include X, NFS
Named Pipes
Communication mechanisms
- Between processes running on the same host
- Pipes are also known as FIFO queues
- Similar to pipes on the command line
- But the persistent is permanent
Symbolic links
- Also commonly referred to as a “soft link” or “symlink”
- Different from “hard link” that replicates inode
- Hard links create a direct reference
- Symbolic links point to the file by name
- Deleting file can leave a dangling link
- Soft links can cross mount points, hard links cannot
- Hard link with
ln, soft link with ln -s
- Soft links work on directories, hard links do not
Kernel Data Structures
- Filesystem acts as an interface to kernel data
- Separate filesystem mounted to /proc
- Can be used to obtain info about system
$ cat /proc/cpuinfo
cpu family : 6
model : 23
model name : Intel(R) Core(TM)2 Duo CPU E8300 @ 2.83GHz
cpu MHz : 1998.000
cache size : 6144 KB
Modifying kernel parameters
Can be used to modify kernel parameters
$ sysctl -a
dev.cdrom.info = drive name: sr0
dev.cdrom.info = drive speed: 48
dev.cdrom.info = Can close tray: 1
dev.cdrom.info = Can open tray: 1
