Core CS

Contiguous Allocation

Contiguous allocation is a file allocation technique where an operating system assigns each file a single, unbroken set of consecutive physical blocks on the disk. Because all data blocks reside side by side, reading or writing a file requires minimal disk head movement, making file access exceptionally fast. However, it requires knowing the final file size in advance and frequently leaves unused, fragmented gaps between allocated spaces.

<h2>Detailed DefinitionContiguous allocation is a method of disk space management in operating systems where every file is assigned a single, continuous sequence of linear disk blocks. Accessing block $i$ of a file simply requires calculating {Starting Block} + i, enabling direct hardware access without navigating pointers or index tables.Key Points<ul><li>Single contiguous span: A file cannot be split across different parts of the disk; all its data blocks must be adjacent.</li><li>Minimal metadata: The directory entry only stores two values: the start block address and the length (number of blocks).</li><li>Dual access support: It naturally supports both sequential access (reading block by block) and direct/random access (jumping straight to a specific block using basic math).</li></ul>Significance<ul><li>Peak I/O speed: It minimizes mechanical disk arm movement (seek time) and rotational latency because the read/write head moves smoothly across adjacent sectors.</li><li>Standard for read-only media: It is widely used in systems where file sizes are fixed and known beforehand, such as optical media (CDs, DVDs) and static embedded firmware.</li></ul>Advantages<ul><li>Fast performance: Sequential reads and writes are extremely fast due to zero head repositioning between adjacent blocks.</li><li>Simple implementation: Directory entries remain small and easy to manage since they only store two integers per file.</li><li>Easy random access: Directly accessing the n-th block requires no traversing; the system calculates the exact disk address instantly.</li></ul>Disadvantages<ul><li>External fragmentation: Over time, creating and deleting files leaves scattered, small free spaces across the disk that may be too small to hold new files, wasting usable storage.</li><li>File growth limitations: Expanding an existing file is difficult. If the block immediately following the file is already occupied, the entire file must either be moved to a larger free space or denied growth.</li><li>Pre-allocation overhead: The system must know or estimate the final file size at creation time, which often leads to either overestimating (wasting space internally) or running out of room.</li></ul></h2>