PCIe Lane Allocation and Sharing: Even if the slot is rated as x4, check whether those lanes are dedicated or shared with other devices on the motherboard. Shared lanes can reduce available bandwidth.
SAS Controller and Firmware: The performance of the SAS controller itself, including its internal buffering, processing capabilities, and firmware optimizations, can introduce bottlenecks even if the PCIe interface is fast.
Host CPU and Memory Subsystem: The system’s CPU and memory bandwidth play a role, especially if RAID calculations or data integrity checks are being done in software. Poor CPU performance or limited memory bandwidth can hinder overall throughput.
Driver and Operating System Overheads: The efficiency of the drivers and the way the operating system handles I/O operations (interrupt handling, queue depth management, etc.) can impact the effective bandwidth.
Cabling and Physical Connection Quality: The quality and length of the SAS cables, as well as the integrity of the physical connections, can introduce latency or errors that slow down data transfer.
Thermal and Power Considerations: Overheating components might throttle performance. Ensure proper cooling and stable power delivery to all components involved.
Real-World Throughput vs. Theoretical Limits: Always account for protocol overhead and the fact that real-world performance (due to command processing, error correction, and queuing delays) can fall short of the theoretical maximum.