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Difficulty: MediumStorage Devices and Interfaces

A technician installs an NVMe M.2 solid-state drive (SSD) rated for sequential read speeds up to 7,000 MB/s7,000\text{ MB/s} into a motherboard's secondary M.2 slot. Upon testing, benchmark software indicates that drive throughput maxes out at approximately 550 MB/s550\text{ MB/s}. Upon reviewing motherboard documentation, the technician notes that the secondary M.2 slot is routed exclusively through the legacy SATA controller rather than PCIe lanes. Which of the following best explains why the drive throughput is limited to this speed?

  1. The M.2 slot interface is bottlenecked by the maximum throughput limitation of the SATA III bus standard.Answer
  2. B
    The system lacks installed AHCI storage drivers, forcing the NVMe drive to fall back to IDE emulation speed.
  3. C
    The drive requires a physical M.2 B-key slot to enable full NVMe protocol bandwidth across PCIe lanes.
  4. D
    The secondary M.2 slot dynamically converts high-speed NVMe commands into SAS protocol packets.

Answer

The M.2 slot interface is bottlenecked by the maximum throughput limitation of the SATA III bus standard.
The correct option identifies that the SATA III bus standard imposes a theoretical maximum speed of 6 Gbps6\text{ Gbps} (roughly 550–600 MB/s550\text{--}600\text{ MB/s} real-world throughput). When an M.2 slot is wired exclusively through the SATA controller, any installed drive operates under these bandwidth constraints regardless of its rated PCIe NVMe speed capabilities.

Step-by-Step Solution

1
Analyze the reported storage throughput metrics.
The drive achieves approximately 550 MB/s550\text{ MB/s} read speeds despite being rated for 7,000 MB/s7,000\text{ MB/s}.
Identifying the observed speed threshold helps pinpoint the exact hardware bus ceiling.
2
Evaluate the underlying bus connectivity of the secondary M.2 slot.
The motherboard documentation indicates the slot is connected via the SATA controller.
SATA III (Revision 3.0) operates at 6 Gbps6\text{ Gbps} maximum signaling rate, which limits effective data throughput to around 550–600 MB/s550\text{--}600\text{ MB/s} after overhead.
3
Differentiate between NVMe PCIe bandwidth and SATA bus limits.
When an M.2 slot operates in SATA mode, NVMe PCIe throughput cannot be achieved because transfer capabilities are constrained by the SATA controller.
Interface bus routing dictates maximum throughput regardless of the installed drive's potential maximum performance.

Key Concept

SATA III vs. NVMe PCIe Storage Bus Bandwidth Limitations
Estimated Time:1m 15s
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Topics

Storage Devices and Interfaces

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