RAID Chunk Size and Workload Strategies

When discussing storage performance, many focus only on RAID levels (0, 1, 5, 6, 10, etc.). However, one of the most overlooked yet critical factors that directly impacts real-world RAID performance is the RAID chunk size. Chunk size defines the smallest unit of data distributed across disks, and this value determines both the degree of disk participation and the resulting I/O pattern. In practice, chunk size can largely dictate RAID read and write performance.

Strategies by Workload Type

Read-Intensive (Sequential Read)

For workloads such as video streaming or backup servers that primarily read large files sequentially, a larger chunk size is advantageous. With larger chunks, each disk can read longer segments of data continuously, reducing unnecessary disk switching and minimizing head movement. The result is more efficient throughput and maximum bandwidth utilization.

Write-Intensive (Random Write)

In environments such as transactional databases or log servers where small, frequent writes dominate, a smaller chunk size delivers better results. Smaller blocks mean more disks participate in each write, maximizing parallelism. Combined with RAID controller caching policies (e.g., write-back), small I/Os can be absorbed and processed efficiently.

Mixed I/O

In virtualization platforms or general-purpose file servers, workloads often combine both sequential and random operations. Here, balance is key. A chunk size that is too small hurts sequential reads, while one that is too large degrades random write performance. Mid-range values, typically 64KB to 128KB, provide a stable compromise. Notably, recommended stripe sizes for VMware VMFS and Hyper-V often fall within this range.

File Size and Disk Participation

Chunk size does more than simply define block size—it directly influences disk participation relative to file size.

For example, storing a 1MB file with a 64KB chunk size requires 16 blocks, which RAID distributes across multiple disks. As a result, multiple disks participate simultaneously in reading or writing that file, maximizing parallelism. In contrast, with a 256KB chunk size, the same file is divided into only 4 blocks. Fewer disks are involved, which increases sequential efficiency but reduces parallel throughput.

Thus, depending on workload and file characteristics, it may be more beneficial to leverage broad disk participation or to optimize for continuous sequential access. This participation difference is a core driver of RAID performance outcomes.

Conclusion

Chunk size is a fundamental parameter that can make or break RAID performance. The right value depends on whether the workload is read-heavy, write-heavy, or mixed. Moreover, the relationship between file size and chunk size dictates disk participation, which in turn determines both I/O parallelism and sequential efficiency. RAID should not be viewed as “set the level and you’re done.” Choosing the proper chunk size is essential to fulfilling RAID’s core purpose: leveraging multiple disks simultaneously to maximize performance.

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