Modelling flash devices with FDR: progress and limits

We present our experience of working with the Failures-Divergence Refinement (FDR) toolkit while extending our modelling of the behaviour of Flash Memory. This effort is a step towards the low-level modelling of data-storage technology that is the target of the POSIX filestore minichallenge. The key objective was to advance previous work presented in [4,2] to cover the full Open Nand-Flash Interface (ONFi) 2.1 model. The previous work covered a sub-model of the mandatory features of ONFi 1.0. The FDR toolkit was used for refinement/ model-checking. In addition to the compression techniques available in FDR, we also experimented with FDR Explorer - an application programming interface (API) that allowed us to get a better picture of FDR performance. This paper summarises the progress we made, and the limits we encountered. We are now able to verify many of the operations in ONFi 2.1 model using full Failures-Divergence refinement checking, rather than just trace refinement. Through the use of compression techniques available in the FDR toolkit and in particular by hiding the events deeper in the model, we were able to get compression of the state-space. The work also reports the number of attempts to compile the full ONFi 2.1 model.