C2J: compulsory compound transaction for journaling file system

In this work, we developed a new technique called Compulsory Compound Journaling (C2J) to increase the coalescing degree of shared updates in journal transactions. C2J introduces intentional delay in waking up the JBD thread to extend the interval during which a transaction has more chance to collect modified blocks. C2J dynamically determines the timing of journal transaction delay by accounting for the workload intensity and the speed of the storage device. We implemented C2J in EXT4. In our evaluation, we confirmed that C2J is effective under various workloads with various storage devices, improving the throughput up to 65\% in the mdtest and reducing the fsync() average latency and tail latency (99.99\%) up to 32\% and 36\%, respectively under varmail workload with 32 threads.