How does increasing queue depth typically affect HDD throughput compared to SSD performance?

How does increasing queue depth typically affect HDD throughput compared to SSD performance?

• A. It represents the number of outstanding I/O requests.
• B. It is the maximum size of a single I/O request.
• C. It is the total data transferred per second.
• D. HDD throughput improves with higher queue depth; SSD gains saturate, showing diminishing returns.

Answer: (D)

Increasing queue depth means more outstanding I/O requests in flight. For HDDs, this helps hide the long latencies from seeking and rotational delays: as you queue more requests, the drive can keep the head moving and the disk spinning while other requests wait, so throughput rises with higher queue depth. But this improvement isn’t limitless—the mechanical nature and the interface eventually bottleneck the drive, so gains taper off at higher depths. SSDs, by contrast, have no moving parts, so they don’t benefit from latency hiding in the same way, and their throughput grows with queue depth only up to the device’s and bus’s bandwidth limits; after that, adding more in-flight requests yields diminishing returns. That combination—HDD throughput improving with higher queue depth, while SSD gains saturate—is the best description.

Increasing queue depth means more outstanding I/O requests in flight. For HDDs, this helps hide the long latencies from seeking and rotational delays: as you queue more requests, the drive can keep the head moving and the disk spinning while other requests wait, so throughput rises with higher queue depth. But this improvement isn’t limitless—the mechanical nature and the interface eventually bottleneck the drive, so gains taper off at higher depths. SSDs, by contrast, have no moving parts, so they don’t benefit from latency hiding in the same way, and their throughput grows with queue depth only up to the device’s and bus’s bandwidth limits; after that, adding more in-flight requests yields diminishing returns. That combination—HDD throughput improving with higher queue depth, while SSD gains saturate—is the best description.