LDJ: Version Consistency Is Almost Free on Commercial Storage Devices

Abstract

In this article, we propose a simple but practical and efficient optimization scheme for journaling in ext4, called lightweight data journaling (LD3). By compressing journaled data prior to writing, LD3 can perform comparable to or even faster than the default ordered journaling (OJ) mode in ext4 on top of both HDDs and flash storage devices, while still guaranteeing the version consistency of the data journaling (DJ) mode. This surprising result can be explained with three main reasons. First, on modern storage devices, the sequential write pattern dominating in DJ mode is more and more high-performant than the random one in OJ mode. Second, the compression significantly reduces the amount of journal writes, which will in turn make the write completion faster and prolong the lifespan of storage devices. Third, the compression also enables the atomicity of each journal write without issuing an intervening FLUSH command between journal data blocks and commit block, thus halving the number of costly FLUSH calls in LD3. We have prototyped our LD3 by slightly modifying the existing ext4 with jbd2 for journaling and also e2fsck for recovery; less than 300 lines of source code were changed. Also, we carried out a comprehensive evaluation using four standard benchmarks and three real applications. Our evaluation results clearly show that LD3 outperforms the OJ mode by up to 9.6x on the real applications.