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A Compression-based Data Consistency Mechanism for File Systems
Dong Hyun Kang, Sang-Won Lee, Young Ik Eom
http://doi.org/10.5626/JOK.2019.46.9.885
Data consistency mechanism is a crucial component in any file system; the mechanism prevents the corruption of data from system crashes or power failures. For the sake of performance, the default journal mode of the Ext4 file system guarantees only the consistency of metadata while compromising with the consistency of normal data. Specially, it does not guarantee full consistency of the whole data of the file system. In this paper, we propose a new crash consistency scheme which guarantees strong data consistency of the data journal mode by still providing higher or comparable performance to the weak default journal mode of the Ext4 file system. By leveraging a compression mechanism, the proposed scheme can halve the amount of write operations as well as the number of fsync() system calls. For evaluation of the performance, we modified the codes related to the jbd2 and compared the proposed scheme with two journaling modes in Ext4 on SSD and HDD. The results clearly confirm that the proposed scheme outperforms the default journal mode by 8.3x times.
An Efficient SLC-buffer Management Scheme for TLC NAND Flash-based Storage
Kirock Kwon, Dong Hyun Kang, Young Ik Eom
http://doi.org/10.5626/JOK.2018.45.7.611
In recent years, almost all consumer devices have adopted NAND flash storage as their main storage, and their performance and capacity requirements are getting higher. To meet these requirements, many researchers have focused on combined SLC-TLC storage consisting of high-speed SLC and high-density TLC. In this paper, we redesign the internal structure of the combined SLC-TLC storage to efficiently manage the SLC region inside the storage and propose a scheme that improves the performance of the storage by employing the I/O characteristics of file system journaling. We implemented our scheme on the real storage platform, the OpenSSD jasmine board, and compared it with the conventional techniques. Our evaluation results show that our technique improves the storage performance by up to 65%, compared with the conventional techniques.
An Efficient Cleaning Scheme for File Defragmentation on Log-Structured File System
Jonggyu Park, Dong Hyun Kang, Euiseong Seo, Young Ik Eom
When many processes issue write operations alternately on Log-structured File System (LFS), the created files can be fragmented on the file system layer although LFS sequentially allocates new blocks of each process. Unfortunately, this file fragmentation degrades read performance because it increases the number of block I/Os. Additionally, read-ahead operations which increase the number of data to request at a time exacerbates the performance degradation. In this paper, we suggest a new cleaning method on LFS that minimizes file fragmentation. During a cleaning process of LFS, our method sorts valid data blocks by inode numbers before copying the valid blocks to a new segment. This sorting re-locates fragmented blocks contiguously. Our cleaning method experimentally eliminates 60% of file fragmentation as compared to file fragmentation before cleaning. Consequently, our cleaning method improves sequential read throughput by 21% when read-ahead is applied.
Analyses of the Effect of System Environment on Filebench Benchmark
Yongju Song, Junghoon Kim, Dong Hyun Kang, Minho Lee, Young Ik Eom
In recent times, NAND flash memory has become widely used as secondary storage for computing devices. Accordingly, to take advantage of NAND flash memory, new file systems have been actively studied and proposed. The performance of these file systems is generally measured with benchmark tools. However, since benchmark tools are executed by software simulation methods, many researchers get non-uniform benchmark results depending on the system environments. In this paper, we use Filebench, one of the most popular and representative benchmark tools, to analyze benchmark results and study the reasons why the benchmark result variations occur. Our experimental results show the differences in benchmark results depending on the system environments. In addition, this study substantiates the fact that system performance is affected mainly by background I/O requests and fsync operations.
Dual Write Buffer Algorithm for Improving Performance and Lifetime of SSDs
Se Jun Han, Dong Hyun Kang, Young Ik Eom
In this paper, we propose a hybrid write buffer architecture comprised of DRAM and NVRAM on SSD and a write buffer algorithm for the hybrid write buffer architecture. Unlike other write buffer algorithms, the proposed algorithm considers read pages as well as write pages to improve the performance of storage devices because most actual workloads are read-write mixed workloads. Through effectively managing NVRAM pages, the proposed algorithm extends the endurance of SSD by reducing the number of erase operations on NAND flash memory. Our experimental results show that our algorithm improved the buffer hit ratio by up to 116.51% and reduced the number of erase operations of NAND flash memory by up to 56.66%.
Improving Performance of I/O Virtualization Framework based on Multi-queue SSD
Tae Yong Kim, Dong Hyun Kang, Young Ik Eom
Virtualization has become one of the most helpful techniques in computing systems, and today it is prevalent in several computing environments including desktops, data-centers, and enterprises. However, since I/O layers are implemented to be oblivious to the I/O behaviors on virtual machines (VM), there still exists an I/O scalability issue in virtualized systems. In particular, when a multi-queue solid state drive (SSD) is used as a secondary storage, each system reveals a semantic gap that degrades the overall performance of the VM. This is due to two key problems, accelerated lock contentions and the I/O parallelism issue. In this paper, we propose a novel approach, including the design of virtual CPU (vCPU)-dedicated queues and I/O threads, which efficiently distributes the lock contentions and addresses the parallelism issue of Virtio-blk-data-plane in virtualized environments. Our approach is based on the above principle, which allocates a dedicated queue and an I/O thread for each vCPU to reduce the semantic gap. Our experimental results with various I/O traces clearly show that our design improves the I/O operations per second (IOPS) in virtualized environments by up to 155% over existing QEMU-based systems.
Hybrid Main Memory based Buffer Cache Scheme by Using Characteristics of Mobile Applications
Chansoo Oh, Dong Hyun Kang, Minho Lee, Young Ik Eom
Mobile devices employ buffer cache mechanisms, just as in computer systems such as desktops or servers, to mitigate the performance gap between main memory and secondary storage. However, DRAM has a problem in that it accelerates battery consumption by performing refresh operations periodically to maintain the stored data. In this paper, we propose a novel buffer cache scheme to increase the battery lifecycle in mobile devices based on a hybrid main memory architecture consisting of DRAM and non-volatile PCM. We also suggest a new buffer cache policy that allocates buffers based on process states to optimize the performance and endurance of PCM. In particular, our algorithm allocates each page to the appropriate position corresponding to the state of the application that owns the page, and tries to ensure a rapid response of foreground applications even with a small amount of DRAM memory. The experimental results indicate that the proposed scheme reduces the elapsed time of foreground applications by 58% on average and power consumption by 23% on average without negatively impacting the performance of background applications.
A Buffer Cache Replacement Algorithm for Considering both Hybrid Main Memory and Storage
PRAM is being considered as a potential successor to DRAM because of its characteristics such as byte-addressability, non-volatility, and high density. To gain its benefits, buffer cache replacement algorithm based on PRAM has been actively studied. However, most of the previous studies on buffer cache replacement algorithm limitedly exploit the byte-level performance of PRAM by focusing its limited lifetime and slower access latency compared to DRAM. In this paper, we propose a novel buffer cache replacement algorithm that fully considers the byte-level performance of PRAM and the performance of secondary storage. To take advantage of small size write on PRAM, proposed scheme keeps pages, which are frequently accessed with a small size write, on PRAM and allows the selective page migration from DRAM to PRAM. As a result, our scheme significantly reduces the number of PRAM writes. Our experimental results indicate for real workloads that our scheme reduces the number of PRAM writes by up to 92% and improves its performance by up to 62% compared to CLOCK.
Data Consistency-Control Scheme Using a Rollback-Recovery Mechanism for Storage Class Memory
Hyun Ku Lee, Junghoon Kim, Dong Hyun Kang, Young Ik Eom
Storage Class Memory(SCM) has been considered as a next-generation storage device because it has positive advantages to be used both as a memory and storage. However, there are significant problems of data consistency in recently proposed file systems for SCM such as insufficient data consistency or excessive data consistency-control overhead. This paper proposes a novel data consistency-control scheme, which changes the write mode for log data depending on the modified data ratio in a block, using a rollback-recovery scheme instead of the Write Ahead Logging (WAL) scheme. The proposed scheme reduces the log data size and the synchronization cost for data consistency. In order to evaluate the proposed scheme, we implemented our scheme on a Linux 3.10.2- based system and measured its performance. The experimental results show that our scheme enhances the write throughput by 9 times on average when compared to the legacy data consistency control scheme.
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