New trends in storage industry suggest that in the near future a majority of the hard disk drive-based storage subsystems will be replaced by solid state drives (SSDs). Database management systems can substantially benefit from the superior I/O performance of SSDs. Although the impact of using SSD in query processing has been studied in the past, exploiting the I/O parallelism of SSDs in query processing and optimization has not received enough attention. In this paper, at first, we show why the query optimizer needs to be aware of the benefit of the I/O parallelism in solid state drives. We characterize the benefit of exploiting I/O parallelism in database scan operators in SAP SQL Anywhere and propose a novel general I/O cost model that considers the impact of device I/O queue depth in I/O cost estimation. We show that using this model, the best plans found by the optimizer would be much closer to optimal. The proposed model is implemented in SAP SQL Anywhere. This model, dynamically defined by a calibration process, summarizes the behavior of the I/O subsystem, without having any prior knowledge about the type and the number of devices which are used in the storage subsystem.