Reducing excessive journaling overhead with small-sized NVRAM for mobile devices

Journaling techniques are widely used to guarantee file system consistency of battery-powered mobile devices such as smartphones and tablets. In a journaling file system, system recovery is facilitated by first writing updated data blocks to a journal area and then periodically writing them to their home locations. However, these duplicated writes degrade the performance and shorten the lifetime of NAND flash storage in mobile devices. In particular, a lightweight database library, which is mainly used to manage application data in mobile devices, is a major cause of excessive journaling because it frequently triggers the costly file synchronization to guarantee the atomicity of transactional execution and thus generates a significant amount of synchronous random write traffic. This paper presents a novel journaling scheme, called Delta Journaling (DJ), to resolve this problem efficiently by using small-sized nonvolatile random access memory (NVRAM). DJ is based on a unique update pattern found in mobile devices, where file system updates are mostly very small. By exploiting the byteaddressable and the nonvolatile characteristics of NVRAM, DJ stores a journal block as a compressed delta in the smallsized NVRAM only when the compressed delta is small enough. Experimental results show that DJ outperforms a traditional journaling file system by up to 16.8 times for synthetic workloads. For a real-world workload, it enhances transaction throughput by 25.5% and 29.2% in ordered and journal modes, respectively, with only 16 MB NVRAM. Also, DJ enhances the lifetime of NAND flash storage by eliminating almost all journal writes without any loss of reliability.