“LOCKED-Free Journaling: Improving the Coalescing Degree in EXT4 Journaling

Abstract

With the recent development of the low-latency storage devices, IO latency is not a critical performance bottleneck of filesystems any more. Instead, CPU Utilization and lock contention have become more critical factors to achieve higher performance. However, EXT4's transaction commit procedure is not suitable for low-latency storage devices due to the presence of the transaction's LOCKED state. In this paper, we first analyze blocked threads that have tried to update filesystem because of LOCKED state and fsync() operation. We then propose an Elimination Transaction Lock-Up scheme that optimizes a transaction commit procedure for low-latency SSDs. With the lock-up elimination scheme, transaction Lock-up overheads from journaling threads can be efficiently eliminated while still ensuring consistency of the EXT4 filesystem.