Willow: A User-Programmable SSD.

We explore the potential of making programmability a central feature of the SSD interface. Our prototype system, called Willow, allows programmers to augment and extend the semantics of an SSD with application-specific features without compromising file system protections. The SSD Apps running on Willow give applications low-latency, high-bandwidth access to the SSD's contents while reducing the load that IO processing places on the host processor. The programming model for SSD Apps provides great flexibility, supports the concurrent execution of multiple SSD Apps in Willow, and supports the execution of trusted code in Willow. We demonstrate the effectiveness and flexibility of Willow by implementing six SSD Apps and measuring their performance. We find that defining SSD semantics in software is easy and beneficial, and that Willow makes it feasible for a wide range of IO-intensive applications to benefit from a customized SSD interface.