A vast majority of smartphones use eMMC (embedded multimedia card) devices as their storage subsystems. Recent studies reveal that storage subsystem is a significant contributor to the performance of smartphone applications. Nevertheless, smartphone applications’ block-level I/O characteristics and their implications on eMMC design are still poorly understood. In this research, we collect and analyze block-level I/O traces from 18 common applications (e.g., Email and Twitter) on a Nexus 5 smartphone. We observe some I/O characteristics from which several implications for eMMC design are derived. For example, we find that in 15 out of the 18 traces majority requests (44.9%-57.4%) are small single-page (4KB) requests. The implication is that small requests should be served rapidly so that the overall performance of an eMMC device can be boosted. Next, we conduct a case study to demonstrate how to apply the implications to optimize eMMC design. Inspired by two implications, we propose a hybrid-page-size (HPS) eMMC. Experimental results show that the HPS scheme can reduce mean response time by up to 86% while improving space utilization by up to 24.2%.