SAS Expander DMD Timer Segmentation for Controller Load Reduction
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Solution Overview
Problem
As storage topology size increases, the system resources required to maintain Device Missing Delay (DMD) timers in controllers also increase, leading to higher solution costs and decreased performance, as well as potential device misidentification due to processing delays.
Innovation Solution
Distributing the load of multiple DMD timers throughout a SAS/SATA topology by incorporating DMD timers in expanders, allowing them to manage devices directly connected, thereby reducing the overhead on the controller and enabling more efficient handling of device disconnections and reconnections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DMD timers are maintained in the controller for all devices in the topology, then device missing handling is reliable, but system resources increase and performance decreases as topology size increases
Solution Approach 1:
The patent divides the DMD timer management function between the controller and expanders. Each expander maintains its own DMD timers for devices directly connected to it, while the controller maintains timers only for devices directly connected to the controller. This segmentation reduces the controller's resource consumption while maintaining reliable device missing handling throughout the topology.
2Ease of manufacture
If a single DMD value is used for the entire topology, then implementation is simple, but different parts of the topology cannot have optimized delay values
Solution Approach 1:
The patent enables each expander to have its own DMD timer with configurable delay values tailored to specific topology sections. This allows different parts of the topology to have optimized delay values according to their specific requirements, while maintaining a consistent and simple implementation approach through the standardized expander structure.
Data Source
AI summary
A SAS expander includes DMD timers for each PHY so that the expander can track disconnected devices directly connected to the expander and signal a SAS controller when the DMD is exceeded. A system including such SAS expanders may reduce the load on the system controller. A controller may recognize expanders capable of tracking DMDs for backwards compatibility.