NV-Journaling: Locality-Aware Journaling Using Byte-Addressable Non-Volatile Memory

Abstract

Modern file systems rely on the journaling mechanism to maintain crash consistency. The use of non-volatile memory (NVM) significantly improves the performance of journaling file systems. However, the superior performance of NVM will increase the likelihood of the journal filling up more often, thereby increasing the frequency of checkpointing. Together with the large amount of random checkpointing I/O found in most use cases, the checkpointing process becomes a new performance bottleneck. This paper proposes NV-Journaling, a strategy that reduces the frequency of checkpointing as well as reshapes the I/O pattern of checkpointing from one of random I/O to that which is more sequential I/O. NV-Journaling introduces fine-grained commits along with a cache-friendly NVM journaling layout that exploits the idiosyncrasies of NVM technology. Under this scheme, only the modified portion of a block, rather than the entire block, is written into the NVM journal device. Doing so significantly reduces checkpoint frequency and achieves better space utilization. NV-Journaling further reshapes the I/O pattern of checkpoint using a locality-aware checkpointing process. Checkpointed blocks are classified into hot and cold blocks. NV-Journaling maintains a hot block list to absorb repeated updates, and a cold bucket list to group blocks by their proximity on disk. When a checkpoint is required, cold buckets are selected such that blocks are sequentially flushed to the hard disk. We built a prototype of NV-Journaling by modifying the JBD2 layer in the Linux kernel and evaluated it using different workloads. Our experimental results show that NV-Journaling can improve performance by up to 4.3× compared to traditional journaling.