Inode Object (in VFS)

 

1. Inode Object (in VFS)

In the Virtual File System (VFS) layer, an inode object is the in-memory representation of a file.

✅ What it represents

• A file (regular file, directory, device, etc.)

• Its metadata (not the actual data)

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📌 Key idea

• Disk → stores inode structure (persistent)

• Memory → VFS creates inode object for active files

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Contents of an inode object

The inode object typically contains:

1. File metadata

• File type (file, directory, device)

• Permissions (read/write/execute)

• Owner (user ID, group ID)

• File size

• Timestamps (access, modify, change)

2. File location info

• Pointers to data blocks (direct/indirect)

3. VFS-specific fields

• Reference count (how many processes use it)

• Locking information

• Pointer to inode operations

• Pointer to file system type

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📌 Important concept

👉 One inode object = one file in memory

Even if multiple processes open the same file:

• They share the same inode object

• But have different file descriptors

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2. Inode Operations

In VFS, each inode has a set of operations (functions) associated with it.

👉 These are called inode operations and define what you can do with that file.

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🧩 Why inode operations exist

Different file systems (ext2, ext3, NFS, etc.) behave differently.

👉 VFS solves this by:

• Defining a common interface

• Letting each file system implement its own version

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⚙️ Common inode operations

Here are the most important ones:

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📂 1. lookup

• Finds a file inside a directory

👉 Example:

/home/user/file.txt

• Lookup is used to find file.txt inside /home/user

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➕ 2. create

• Creates a new file in a directory

👉 Used when:

open("file", O_CREAT)

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❌ 3. unlink

• Removes a file (deletes a name)

👉 Important:

• Decreases link count

• Deletes file only if count = 0

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📁 4. mkdir

• Creates a directory

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🗑️ 5. rmdir

• Deletes a directory (must be empty)

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🔗 6. link

• Creates a hard link

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🔗➡️ 7. symlink

• Creates a symbolic link

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✏️ 8. rename

• Renames a file or directory (atomic operation)

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📊 9. getattr

• Gets file metadata (like stat())

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✍️ 10. setattr

• Changes metadata (permissions, size, etc.)

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3. How VFS Uses Inode + Operations

🔄 Flow example: opening a file

1. Path is given:

   /home/user/file.txt
   

2. VFS:

   • Uses lookup() step-by-step

   • Finds corresponding inode objects

3. Once inode is found:

   • Uses inode operations for further actions

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Key Insight

👉 Inode = data (what the file is)
👉 Inode operations = behavior (what you can do with it)

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4. Simple Analogy

Think of inode like this:

Concept	Real-world analogy
Inode object	File record in a library
Data blocks	Pages of the book
Inode operations	Actions (read, write, delete)
VFS	Librarian managing all books

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5. Key Points to Remember

• Inode object is in-memory representation of a file

• Stores metadata, not file name

• File name is stored in directory

• Inode operations define file behavior

• VFS provides uniform interface across file systems

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 Summary

• Inode object: Represents a file in memory with metadata and pointers

• Inode operations: Functions that define actions like create, delete, lookup, etc.

• VFS uses them to provide a common interface across different file systems