Linux File System and Processes

Linux Filesystem

What is it? 

  • The Linux Filesystem is the way that data is stored to the harddrive
  • Also contains information regarding processes and connected devices
  • Root directory is /. User directories are located in /home/<username>

What is it? 

  • Often called Virtual Filesystem because it is an abstraction layer for FS like EXT4, FAT32, etc. 
  • Why is it done this way?
    • Kernel can support various implementation
    • Easy interoperation between various file system types

What is it? 

  • Often called Virtual Filesystem because it is an abstraction layer for FS like EXT4, FAT32, etc. 
  • Why is it done this way?
    • Kernel can support various implementation
    • Easy interoperation between various file system types

Application

Write()

Read()

sys_write

sys_read

Firmware

EXT4, FAT32 ...

Hierarchy

  • /bin - Essential cmd line utilities
  • /boot - Boot loader files
  • /dev - Physical and Virtual Device Files
  • /etc - Static configuration files
  • /home - User home directories
  • /lib(64) - Library files
  • /media - Mount points for removable devices
  • /mnt - Temporarily mounted FS
  • /opt - Additional Software 
  • /proc - Virtual Filesystem (we will get more into this) 

Hierarchy

  • /root - Home directory for root user
  • /run - Run time variable data
  • /sbin - System binaries (fsck, init, route)
  • /srv - Served data (e.g. FTP, HTTP servers)
  • /sys - Information about drives, kernel
  • /tmp - Temporary FS (memory backed)
  • /usr - Multiuser binaries
  • /var - Variable Files

File System Implementations

  • ext4 - Extended File System version 4 
    • Used by most Linux distributions, originally developed in 1993
    • Slides will be covering this file system primarily
  • XFS - Extended File System
    • High performance 64 bit journaling FS
    • Default for RedHat/CentOS
  • SquashFS
    • Read-only Filesystem for low-memory devices
  • JFS
    • 64 bit journaling Filesystem
    • Default for AIX

Filesystem Layout

Disk

Partition

Block

Block

Block

Block

Filesystem Layout

  • Block - basic unit of a filesystem 
    • Boot Block
    • Superblock
    • i-node table
    • Data block
  • Partition - Where filesystems are contained
  • Disk - Contains multiple partitions

Filesystem Layout

  • Boot block
    • First block in the file system
    • Used to help the OS boot
  • Superblock
    • Follows the boot block
    • Stores metadata for the file system 
      • Number of blocks 
      • Size of Blocks 
      • Size of i-node table
      • type of file system 
      • etc

Filesystem Layout

  • I-node table
    • List of index nodes 
    • Contains metadata about a file
  • Data block - well... it's exactly as it sounds

How to View Filesystem Metadata

I-nodes (Inodes)

  • Used to represent files and directories
  • Contains metadata about the file such as size, physical location, owner, group, etc.
  • Files are assigned an I-node number on creation
    • Unique identifier to help with indexing
  • The number of i-nodes on a system is fixed

https://en.wikipedia.org/wiki/Inode_pointer_structure

Directories

  • Used to represent files and directories
  • Contains metadata about the file such as size, physical location, owner, group, etc.
  • Files are assigned an I-node number on creation
    • Unique identifier to help with indexing
  • The number of i-nodes on a system is fixed

A Very Deep Dive

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

Explore your Linux FS

See what you can discover about your user directory. Based on what we found. 

Processes

What are they?

  • Processes are running applications. 
  • They are the fundamental concept for working with programs

/proc

  • Otherwise known as the procfs
  • In-memory file system containing details about processes
  • Contains information like memory, stack, addresses, referenced shared libraries, etc. 

/proc exploration

  • Run the command ls /proc
  • Gain familiarity by navigating around the /proc path
  • What do you think is in /proc/cmdline?
  • See /proc/cpuinfo and /proc/meminfo
  • See man 5 proc for more info

/proc/<pid>

  • Contains all the details about a running application
    • cmdline - the command line used to run the process
    • cwd - the current working directory
    • environ - details about the working environment
    • exe - a symbolic link to the actual binary
    • fd - A subdirectory containing any file descriptors used by the program
    • maps - a list of anything mapped into the process
    • status - Displays information  about the process
    • task - A subdirectory containing threads of the process

What's also useful about it?

  • Can read/write kernel settings via the shell from procfs as if they were on disk
  • Can also use a standard API to configure the procfs as well
    • setrlimit 
    • getrlimit 
    • prlimit