Mitigating Log Cost through Non-Volatile Memory and Checkpoint Optimization

The sudden power failure or system crash can result in file system inconsistency upon updating permanent user data or metadata to their home locations in disk layout, an issue known as crash-consistency problem. Most existing file systems leverage some kind of consistency techniques such as write-ahead logging(WAL), copy-on-write(COW) to avoid this situation. Ext4 file system ensures the consistency of persistent operations through transaction as well as journaling mechanism. However, it is required to write file system metadata to disk twice. The metadata has the features with small granularity, big quantity and high repetition, which degrades the performance of program and also shortens the lifetime of flash-based SSD. This paper is proposed to employ non-volatile memory(NVM) as an independent log partition, which can be accessed through load/store interface directly. Furthermore, we optimize disk write operations by using reverse scan while checkpointing in order to reduce the repeated metadata updates to the same data block. The preliminary experimental results show that the performance can be improved up to 50% on HDD, and 23% on SSD for heavy-write workloads when using NVM as the external journal partition device and the number of write operations can be reduced significantly after using reverse scan checkpoint technique.