wrJFS: A Write-Reduction Journaling File System for Byte-addressable NVRAM

Non-volatile random-access memory (NVRAM) becomes a mainstream storage device in embedded systems due to its favorable features, such as small size, low power consumption, and short read/write latency. Unlike dynamic random access memory (DRAM), NVRAM has asymmetric performance and energy consumption on read/write operations. Generally, on NVRAM, a write operation consumes more energy and time than a read operation. Unfortunately, current mobile/embedded file systems, such as EXT2/3 and EXT4, are very unfriendly for NVRAM devices. The reason is that current mobile/embedded file systems employ a journaling mechanism for increasing its data reliability. Although a journaling mechanism raises the safety of data in a file system, it also repeatedly writes data to a data storage while data is committed and checkpointed. Though several related works have been proposed to reduce the amount of write traffic to NVRAM, they still cannot effectively minimize the write amplification of a journaling mechanism. Such observations motivate us to design a two-phase write reduction journaling file system called wrJFS. In the first phase, wrJFS classified data into two categories: Metadata and user data. As the size of metadata is usually very small (few bytes), byte-enabled journaling strategy will handle metadata during commit and checkpoint stages. In contrast, the size of user data is very large relative to metadata; thus, user data will be processed in the second phase. In the second phase, user data will be compressed by hardware encoder to reduce the write size and managed compressed-enabled journaling strategy to avoid the write amplification on NVRAM. Moreover, we analyze the overhead of wrJFS and show that the overhead is negligible. According to the experimental results, the proposed wrJFS outperforms other journaling file systems even though the experiments include the overhead of data compression.

1 Introduction

As byte-addressable non-volatile RAM (NVRAM), such as phase-change memory (PCM) and 3D XPoint  [2], has great potential to be a mainstream storage device in embedded/mobile systems, a conventional file system should be redesigned for fulfilling characteristics of a nice file system for a byte-addressable NVRAM device. A nice file system for a byte-addressable NVRAM device should shun the possibility of causing write amplification1.

Some processes perform specific operations that result in rewriting data to storage more than once.

because a write operation consumes more time and energy than a read operation on a byte-addressable NVRAM device. Unfortunately, for the sake of data safety, modern file systems employ a journaling mechanism that writes the same data twice to both the journal and data area of a storage space. As a result, although many write-reduction solutions [3], [4], [5], [6], [7] have been proposed to reduce the size of written data on a byte-addressable NVRAM device in the past decades, these excellent solutions cannot eliminate the write amplification caused by a journaling mechanism. Such observations motivate us to design a write-reduction journaling mechanism that not only shuns the overhead of write amplification but also reduces the size of a write operation by cooperating with write-reduction strategies.