Linked Allocation: How It Works and When to Use It

Linked allocation is a method of storing files where each file is a linked list of disk blocks. Each block contains data and a pointer to the next block, allowing files to be stored non-contiguously on disk.

How It Works

Linked allocation stores a file as a chain of disk blocks linked together by pointers. Imagine a treasure hunt where each clue leads you to the next location; similarly, each block points to the next block of the file. This means the file's blocks can be scattered anywhere on the disk, not necessarily next to each other.

This method avoids the problem of finding a large continuous space for a file. When reading a file, the system starts at the first block and follows the pointers from one block to the next until the entire file is read. However, random access is slower because you must follow the chain from the start to reach a specific block.

Example

This example simulates linked allocation by creating a linked list of file blocks, each holding data and a pointer to the next block.

python

class FileBlock:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedFile:
    def __init__(self):
        self.head = None

    def add_block(self, data):
        new_block = FileBlock(data)
        if not self.head:
            self.head = new_block
        else:
            current = self.head
            while current.next:
                current = current.next
            current.next = new_block

    def read_file(self):
        current = self.head
        contents = []
        while current:
            contents.append(current.data)
            current = current.next
        return ''.join(contents)

# Create a linked file and add blocks
file = LinkedFile()
file.add_block('Hel')
file.add_block('lo, ')
file.add_block('World!')

print(file.read_file())

Output

Hello, World!

When to Use

Linked allocation is useful when files need to grow dynamically and disk space is fragmented. It works well for sequential access files like logs or streaming data, where reading happens in order. It avoids wasted space from fixed-size allocation and handles files that change size easily.

However, it is less efficient for random access because you must follow pointers from the start. It is also vulnerable to pointer corruption, which can break the chain. Modern systems often use other methods but linked allocation is a simple, flexible approach for certain use cases.

Key Points

• Files are stored as linked lists of disk blocks.
• Each block contains data and a pointer to the next block.
• Blocks can be scattered anywhere on disk, avoiding fragmentation issues.
• Sequential access is fast; random access is slow.
• Simple and flexible but vulnerable to pointer corruption.

Key Takeaways

Linked allocation stores files as chains of linked disk blocks with pointers.

It allows files to be stored non-contiguously, avoiding large free space requirements.

Best suited for sequential file access and files that grow dynamically.

Random access is slow because blocks must be read in order.

Pointer corruption can break the file chain, so reliability is important.