Chapter 4. Introduction to File System [4 Hrs (Assumed)]

• In Linux everything is a file.
• When file is opened, OS uses File System to load it from the underlying storage.
• Filesystem governs the file organization and access.
• Without FS, apps could access storage in incompatible ways (like: simultaneous write, reading and writing together) leading to cascade of errors (like: resource contention, data corruption and then data loss).
• The Filesystem Hierarchy Standard (FHS) defines the directory structure and contents.

Key directories in linux are:

• /: Root directory of entire Filesystem Hierarchy
• /bin: Essential command binaries (like: ls, cat, cp)
• /sbin: Essential system binaries (like: fsck, init, route)
• /dev: Device files (like: /dev/null, /dev/sda1, /dev/random)
• /etc: Host specific system-wide configuration files.
• /home: User home directories
• /usr: Secondary hierarchy for read only user data; this contains majority of multi-user applicaitons and utilities.
• /var: Variable data files like logs, spool etc.

References

• Wikipedia: https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
• FHS reference: https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html

Filesystem Architecture

• Logical File System:
  • Facilitates open, close, and write actions on files.
• Virtual Filesystem:
  • Provides standardized interface, allowing different filesystems to coexist, and function.
• Physical Filesystem:
  • Responssible for space management, storage of data and retrieval.

Absolute and relative path

Absolute path is the location of file or directory specified from the root directory.

• example: /home/lisa/file.py

Relative path is the location of file or directory specified from the current directory.

• example if current directory is /home/lisa/, relative path is ./file.py

Useful commands

• pwd: Print working directory.
• realpath: Prints the absolute path given a relative path.

XFS vs ext4

XFS:

• 64-bit journaling file systems
• Better performance with large files.
• Maximum file size is 8EiB.
• Not compatible with ext FS.
• Capable of snapshot
• Excels at parallel I/O operations.

ext4:

• Journaling file system
• Better performance with small files
• Can store up to 16TB
• Backward compatible wit hext3 and ext2
• Better security features

Reference: https://blog.purestorage.com/purely-educational/xfs-vs-ext4-which-linux-file-system-is-better/

Utilities:

fsck: It refers to FileSystem Consistency checK.

Syntax:

    fsck [OPTIONS] path/to/device

Options:

• -N: Just print what would be done without doing anything.
• -n: Assume no to every action, so checks filesystem without correcting anything.

blkid: It is used to locate and print block device’s attributes. Syntax:

    blkid path/to/device

lsblk: List block devices. Syntax

    lsblk [OPTIONS] [device]

if device is omitted, it shows all block devices.