Home

Contents

Structure

File System Types

Permissions

Mounts

🏗️ Structure

A file system consists of 3 layers

1. Logical File System

The Logical File System acts as the interface between the user applications and the file system itself. It facilitates essential operations such as opening, reading, and closing files. Essentially, it serves as the user-friendly front-end, ensuring that applications can interact with the file system in a way that aligns with user expectations.

1. Virtual File System

The Virtual File System (VFS) is a crucial layer that enables the concurrent operation of multiple instances of physical file systems. It provides a standardized interface, allowing different file systems to coexist and operate simultaneously. This layer abstracts the underlying complexities, ensuring compatibility and cohesion between various file system implementations.

1. Physical File System

The Physical File System is responsible for the tangible management and storage of physical memory blocks on the disk. It handles the low-level details of storing and retrieving data, interacting directly with the hardware components.

Characteristics of a File System

• Space Management: How the data is stored on a storage device. Pertaining to the memory and fragmentation practices applied in it.
• Filename: A file system may have certain restrictions on file names such as length and special characters.
• Directory: The directories/folders may store files in a linear or hierarchical manner while maintaining an index table of all the files contained in that directory or subdirectory.
• Metadata: For each file stored, the file system stores various information about that file’s existence such as its data length, its access permissions, device type, modified date-time, and other attributes.
• Utilities: File systems provide features for initializing, deleting, renaming, moving, copying, backup, recovery, and control access of files and folders.
• Design: Due to their implementations, file systems have limitations on the amount of data they can store.

Some important terms

Journaling - Journaling file systems keep a log called the journal that keeps track of the changes made to a file, but not yet permanently committed to the disk. In case of system failures, lost changes can be brought back using the journal.

Versioning - Version file systems store previously saved versions of a file. Backups are created from storing copies of the file based on previous commits in a timely manner.

Inode - The index node is the representation of any file or directory based on the parameters such as size permission, ownership, and location of the file.

🗄️ File System Types

1. ext (Extended File System): The first file system specifically designed for linux, implemented in 1992.
2. ext2: A non-journaling file system that is preferred to be used with flash drives and SSDs.
3. Xiafs: A file system less powerful and functional than ext2 that it is no longer used anywhere.
4. ext3: A reliable file system and unlike ext2, it prevents long delays at system boot if the file system is in an inconsistent state after an unclean shutdown.
5. JFS: JFS performs well under different kinds of load but is not commonly used anymore due to the release of ext4 in 2006 which gives better performance.
6. ReiserFS

Despite its earlier issues, it has tail packing as a scheme to reduce internal fragmentation. It uses a B+ Tree that gives less than linear time in directory lookups and updates. It was the default file system in SUSE Linux till version 6.4, until switching to ext3 in 2006 for version 10.2.

1. XFS

Used as the default file system for many Linux distributions. It provides features like snapshots, online defragmentation, sparse files, variable block sizes, and excellent capacity. It also excels at parallel I/O operations.

1. SquashFS: Developed in 2002, this file system is read-only and is used only with embedded systems where low overhead is needed.
2. Reiser4: It is an incremental model to ReiserFS.

However, it is not widely adapted or supported on many Linux distributions.

1. ext4

The fourth ext developed in 2006, is a journaling file system. It has backward compatibility with ext3 and ext2 and it provides several other features, some of which are persistent pre-allocation, unlimited number of subdirectories, metadata checksumming and large file size. ext4 is the default file system for many Linux distributions and also has compatibility with Windows and Macintosh.

1. btrfs (Better/Butter/B-tree FS)

btrfs provides many features such as snapshotting, drive pooling, data scrubbing, self-healing and online defragmentation. It is the default file system for Fedora Workstation.

1. bcachefs

This is a copy-on-write file system that was first announced in 2015 with the goal of performing better than btrfs and ext4. Its features include full filesystem encryption, native compression, snapshots, and 64-bit check summing.

1. Others

Linux also has support for file systems of operating systems such as NTFS and exFAT, but these do not support standard Unix permission settings. They are mostly used for interoperability with other operating systems.

<aside> ❓ ext4 has been the main file system developers use despite other systems being better such as btrfs and XFS; but why?

</aside>

EXT4

ext4 was designed to be backward compatible with ext3 and ext2. It’s better than the previous generations in the following ways:

• It provides a large file system.
• Utilizes extents that improve large file performance and reduces fragmentation.
• Provides persistent pre-allocation which guarantees space allocation and contiguous memory.
• Delayed allocation improves performance and reduces fragmentation by effectively allocating larger amounts of data at a time.
• It uses HTree indices to allow unlimited number of subdirectories.
• Performs journal checksumming which allows the file system to realize that some of its entries are invalid or out of order after a crash.
• Support for time-of-creation timestamps and improved timestamps to induce granularity.
• Transparent encryption.
• Allows cleaning of inode tables in background which in turn speeds initialization. The process is called lazy initialization.
• Enables writing barriers by default. Which ensures that file system metadata is correctly written and ordered on disk, even when write caches lose power.

Limitations

Ext4 does not guarantee the integrity of your data. If the data is corrupted while already on disk then it has no way of detecting or repairing such corruption.

It cannot secure deletion of files, which is supposed to cause overwriting of files upon deletion. This results in sensitive data ending up in the file-system journal.

Comparisons

XFS performs highly well for large filesystems and high degrees of concurrency. So XFS is stable, yet there’s not a solid borderline that would make you choose it over ext4 since both work about the same. Unless you want a file system that directly solves a problem of ext4 like having capacity > 50TiB.

Despite btrfs offering features like multiple device management, per-block checksumming, asynchronous replication and inline compression, it does not perform the best in many common use cases as compared to ext4 and XFS. Several of its features can be buggy and result in reduced performance and data loss.

📝 Permissions

The ls command along with its -l (for long listing) option will show you metadata about your Linux files, including the permissions set on the file.

$ ls -l

drwxr-xr-x. 4 root root    68 Jun 13 20:25 tuned
-rw-r--r--. 1 root root  4017 Feb 24  2022 vimrc

Breakdown of drwxr-xr-x:

• d - This indicates that the file is a directory.
• rwx - The first three indicates that the User owner has read, write, and execute permissions.
• r-x - The second three indicates that the Group owner only has read and execute permissions.
• r-x - The last three indicates that Others also has read and execute permissions.
• root - The first root indicates that the user root owns the directory.
• root - The second root indicates that the group root owns the directory.

Octal Values

Octal values are numbers that represents permissions. Each permission has a numeric value assigned to it.

• r (read): 4
• w (write): 2
• x (execute): 1

For example: If a file has the permissions of drwxr-xr-x the octal value would be 755.

• User (rwx): 4+2+1 = 7
• Group (r-x): 4+0+1 = 5
• Others (r-x): 4+0+1 = 5

Here is a table of values:

| --- | --- | --- |

Changing Permissions

There is two ways to change file permissions, symbolic mode or octal values

Symbolic mode

Symbolic mode is a procedural way to change permissions. Meaning it performs the instructions you give. To change permissions using symbolic mode, follow the equation below:

Users(u) or Group(g) or Others(o) +- read(r) or write(w) or execute(x)

🏔️ Mounts