L-7.4: Contiguous Allocation in Operating system | Advantages & Disadvantages

Summary

This video provides a clear explanation of contiguous allocation, a method used in memory management where data is stored in adjacent blocks. It simplifies file access by placing files in continuous storage locations, offering advantages like easy implementation and excellent read performance. However, it also comes with disadvantages, including fragmentation (both internal and external) and difficulty in expanding files when contiguous space is not available. The video aims to help students understand this concept for exams and practical applications in computer systems.

Takeaways

• 😀 Contiguous allocation involves storing files in consecutive blocks of memory or storage, meaning the file data is placed next to each other.
• 😀 It is similar to how students sit in a classroom, with each student (or file) placed next to the other, roll number by roll number.
• 😀 The directory contains information about files, including their starting block and length, indicating how much memory the file occupies.
• 😀 Files are divided into blocks, and each block is assigned to a contiguous section of memory, ensuring efficient storage access.
• 😀 An advantage of contiguous allocation is that it’s easy to implement because no pointers or indices are required to manage file locations.
• 😀 Contiguous allocation provides excellent read performance, especially for sequential and direct access to file data.
• 😀 For sequential access, the system can simply read through blocks one by one, reducing the need to search across different locations.
• 😀 Direct access is efficient because, once the starting block is known, the system can quickly calculate the position of subsequent blocks.
• 😀 A major disadvantage of contiguous allocation is fragmentation, which can occur in two forms: internal fragmentation and external fragmentation.
• 😀 Internal fragmentation happens when a file doesn’t perfectly fit into a block, wasting the remaining space in that block.
• 😀 External fragmentation occurs when free spaces are scattered across the disk, preventing new files from being stored contiguously, even if there’s enough total free space.
• 😀 Contiguous allocation also makes it difficult to grow a file if sufficient contiguous space is not available next to the file’s current location.
• 😀 Overall, contiguous allocation is best suited for systems with predictable, static file sizes and where the need to expand or edit files is limited.

Q & A

What is contiguous allocation in memory management?

Contiguous allocation is a memory management technique where files are stored in adjacent (contiguous) blocks within the storage, meaning that each file occupies a continuous set of memory locations.

Why is the term 'contiguous' used in the context of memory allocation?

The term 'contiguous' refers to the fact that the blocks of memory or disk where the data is stored are adjacent to each other, without gaps in between, much like students sitting in consecutive seats according to their roll numbers.

How is the directory used in contiguous allocation?

In contiguous allocation, the directory stores metadata about the files, such as the file name, starting block, and the number of blocks the file occupies. This helps in tracking the location of each file in the storage.

What is the role of 'length' in the contiguous allocation example?

The 'length' refers to the number of blocks a file occupies in memory. For instance, if a file occupies 3 blocks, the length is 3, and the file will be stored contiguously across those 3 blocks.

What are the key advantages of contiguous allocation?

The main advantages of contiguous allocation include its simplicity (easy to implement), excellent read performance (both sequential and direct access are fast), and the straightforward way files are stored in memory.

How does contiguous allocation improve read performance?

Contiguous allocation improves read performance because when data is stored sequentially, the read-write head of the disk can simply move across the contiguous blocks, reducing seek time and allowing faster data access.

What is the difference between sequential and direct access in contiguous allocation?

In sequential access, data is read in order (e.g., blocks 6 to 10), while in direct access, a specific block (e.g., block 9) can be quickly accessed once the starting block is reached, due to the contiguous arrangement of the blocks.

What is fragmentation in the context of contiguous allocation?

Fragmentation in contiguous allocation occurs when there is unused space either within blocks (internal fragmentation) or between blocks (external fragmentation), making it difficult to store or expand files efficiently.

What is the main issue with external fragmentation in contiguous allocation?

External fragmentation occurs when there is free space available but it is scattered, not contiguous. This makes it impossible to allocate large files or expand existing files, even though there may be enough total free space in the system.

Why is it difficult to grow a file in contiguous allocation?

It is difficult to grow a file in contiguous allocation because if the space after a file is filled, there may not be enough contiguous space to expand the file. If the necessary space is not available next to the existing file, it cannot be extended without reallocation.