Compare and Contrast Between Linked and Indexed Disk Allocation Strategies

Disk allocation strategy determines how a file’s logical blocks are placed on secondary storage and how the operating system later finds them during reads and writes.2 Two classic non-contiguous methods are linked allocation and indexed allocation.2 Both were designed to avoid external fragmentation that affects contiguous schemes, because file blocks can be scattered anywhere on disk.2

The essential difference is structural. In linked allocation, each file is a chain of blocks, and each block stores a pointer to the next one.2 In indexed allocation, each file has an index block containing an array of pointers, where the $i$th entry identifies the $i$th data block of the file.2 This design choice strongly affects access time, growth behavior, metadata overhead, reliability, and suitability for workloads.2

A rigorous comparison therefore centers on four questions: how blocks are located, how efficiently files grow, how much metadata is consumed, and how well sequential versus direct access is supported.3

Footnotes

1. File Allocation Methods - GeeksforGeeks - Overview of contiguous, linked, and indexed allocation with advantages and disadvantages. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

2. Directory Structure and File Allocation Methods - Academic discussion of file allocation strategies, access behavior, and fragmentation properties. ↩ ↩² ↩³ ↩⁴

3. FILE ALLOCATION METHODS.pptx ........... | PPTX - Summarizes direct access, pointer overhead, small-file inefficiency, and corruption risks. ↩ ↩²

4. 22 File Structure, Disk Scheduling - MPI-SWS Courses - Operating systems lecture notes covering linked files, FAT variation, indexed files, and multilevel indexing trade-offs. ↩ ↩²

Conceptual model of linked allocation

In linked allocation the directory typically stores the first block of the file, and sometimes the last block as well. Every data block contains user data plus a pointer to the next block in the file; the last block contains a null marker such as $-1$.2 Because the next block may be anywhere on disk, the method is highly flexible for dynamic file growth.2

A file with logical blocks $B_0, B_1, B_2, B_3$ may physically appear as scattered disk blocks such as $9 \to 16 \to 1 \to 10$. To reach logical block $B_3$, the system must start at the first block and follow the pointer chain through the preceding blocks.2

Conceptual model of indexed allocation

In indexed allocation the directory stores the address of a dedicated index block.2 That index block holds block addresses for the file’s data, so logical block $i$ can be located by reading entry $i$ from the index and then accessing the corresponding data block.2 This enables direct access in addition to sequential access.2

The trade-off is that the index block itself consumes space and may become inefficient for very small files, because even a short file can require a whole block of metadata.2 For larger files, systems may need linked index blocks, multilevel indexes, or combined schemes such as those used in UNIX-style inode designs.

Footnotes

1. File Allocation Methods - GeeksforGeeks - Overview of contiguous, linked, and indexed allocation with advantages and disadvantages. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

2. FILE ALLOCATION METHODS.pptx ........... | PPTX - Summarizes direct access, pointer overhead, small-file inefficiency, and corruption risks. ↩ ↩² ↩³

3. 22 File Structure, Disk Scheduling - MPI-SWS Courses - Operating systems lecture notes covering linked files, FAT variation, indexed files, and multilevel indexing trade-offs. ↩ ↩²

4. Directory Structure and File Allocation Methods - Academic discussion of file allocation strategies, access behavior, and fragmentation properties. ↩ ↩² ↩³

Detailed comparison across design criteria

The most important contrast is access behavior. Linked allocation is efficient for sequential access because the next block pointer is immediately available after reading the current block.2 However, it performs poorly for random access since the operating system must traverse prior links to locate block $k$.2 Indexed allocation directly addresses this weakness by storing all pointers together, allowing immediate lookup of the needed block number.2

A second difference is metadata overhead. Linked allocation embeds one pointer in every data block, reducing usable payload in each block.2 If block size is $b$ bytes and pointer size is $p$ bytes, the effective data capacity per linked block is $b-p$. Indexed allocation instead centralizes pointer storage in one or more index blocks.2 This usually gives cleaner access semantics, but for small files a whole index block may be mostly empty, making space utilization worse than linked allocation.2

A third contrast is fault sensitivity. If a pointer in a linked chain is corrupted, the remainder of the file may become unreachable because the chain is broken. If an index block in indexed allocation is damaged, many or all file block addresses may be lost at once because the map is centralized. Thus, linked allocation risks chain breakage, while indexed allocation concentrates risk in the index structure.

Finally, growth and scalability differ. Linked allocation is naturally elastic: the file grows by appending another block anywhere free space exists.2 Indexed allocation also permits non-contiguous growth, but scalability depends on index design. A single fixed-size index block limits the maximum file size it can describe unless linked or multilevel indexes are used.

Footnotes

1. File Allocation Methods - GeeksforGeeks - Overview of contiguous, linked, and indexed allocation with advantages and disadvantages. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

2. 22 File Structure, Disk Scheduling - MPI-SWS Courses - Operating systems lecture notes covering linked files, FAT variation, indexed files, and multilevel indexing trade-offs. ↩ ↩² ↩³

3. Directory Structure and File Allocation Methods - Academic discussion of file allocation strategies, access behavior, and fragmentation properties. ↩ ↩² ↩³

4. FILE ALLOCATION METHODS.pptx ........... | PPTX - Summarizes direct access, pointer overhead, small-file inefficiency, and corruption risks. ↩ ↩² ↩³ ↩⁴

Analytical comparison table

From a performance perspective, if accessing logical block $k$ in a linked file requires traversing $k$ pointers, the lookup cost behaves approximately like $O(k)$, whereas indexed lookup to the pointer entry is effectively $O(1)$ at the indexing level, ignoring physical I/O delays and cache behavior.2 This asymmetry explains why indexed allocation is preferred when applications frequently jump to arbitrary positions in files.

$$\text{Linked access to block }k \approx O(k), \qquad \text{Indexed pointer lookup to block }k \approx O(1)$$

Footnotes

1. File Allocation Methods - GeeksforGeeks - Overview of contiguous, linked, and indexed allocation with advantages and disadvantages. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶

2. Directory Structure and File Allocation Methods - Academic discussion of file allocation strategies, access behavior, and fragmentation properties. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

3. FILE ALLOCATION METHODS.pptx ........... | PPTX - Summarizes direct access, pointer overhead, small-file inefficiency, and corruption risks. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

4. 22 File Structure, Disk Scheduling - MPI-SWS Courses - Operating systems lecture notes covering linked files, FAT variation, indexed files, and multilevel indexing trade-offs. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

Conclusion

Linked and indexed disk allocation strategies solve the same high-level problem—how to store files without requiring contiguous disk space—but they optimize different priorities.2 Linked allocation emphasizes simplicity and easy incremental growth, making it well suited to sequential workloads and unpredictable file expansion.2 Indexed allocation emphasizes fast logical block lookup and support for direct access, making it more versatile for general-purpose file systems despite its metadata cost.2

In concise comparative terms:

• Choose linked allocation when sequential access and append flexibility matter more than random access.2
• Choose indexed allocation when direct access, structured lookup, and broader workload support are more important.2

For modern systems, pure linked allocation is comparatively rare as a primary strategy, while indexed or hybrid indexed approaches remain foundational because they better support diverse access patterns and large file management.

Footnotes

1. File Allocation Methods - GeeksforGeeks - Overview of contiguous, linked, and indexed allocation with advantages and disadvantages. ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Directory Structure and File Allocation Methods - Academic discussion of file allocation strategies, access behavior, and fragmentation properties. ↩ ↩²

3. 22 File Structure, Disk Scheduling - MPI-SWS Courses - Operating systems lecture notes covering linked files, FAT variation, indexed files, and multilevel indexing trade-offs. ↩ ↩² ↩³

4. FILE ALLOCATION METHODS.pptx ........... | PPTX - Summarizes direct access, pointer overhead, small-file inefficiency, and corruption risks. ↩