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Eager Synching: A Selective Logging Strategy for Fast fsync() on Flash-Based Android Devices
(2017-04-01)
Chang, Li-Pin
;
Sung, Po-Han
;
Chen, Po-Tsang
;
Chen, Po-Hung
;
資訊工程學系
Flash storage has been a standard component in Android devices. Recent research has reported that application data management in Android involves frequent fsync() operations. The current fsync() implementations, including those of ext4 and F2FS, have several common drawbacks. Specifically, ext4 commits a transaction every time to sync a file, whereas F2FS commits a checkpoint to sync a directory. Committing a transaction or checkpoint flushes all dirty data from the page cache to the flash storage via many small, random block write requests. The resultant high I/O frequency and excessive write traffic cause a high fsync() latency. This study presents an efficient fsync() method, called eager synching, which is based on a simple idea: write less, and write sequentially. To sync a file, eager synching writes only a subset of all dirty data in the page cache to a sequential log space using a few sequential block write requests. It does not involve transaction or checkpoint committing. We successfully implemented eager synching in ext4 and F2FS, and our experimental results show that, compared with the original fsync() methods of ext4 and F2FS, eager synching reduced the average and maximum fsync() latencies by up to 72% and 91%, respectively, block-level write traffic by up to 35%, and I/O frequency by up to 66%. Through enhanced crash recovery procedures, eager synching can successfully recover all previously synched files while still guaranteeing the file system integrity. We also conducted live application replays using the proposed eager synching approach and observed that this approach significantly improved the application frame updating rate and application execution time.
Stable Greedy: Adaptive Garbage Collection for Durable Page-Mapping Multichannel SSDs
(2016)
Chang, Li-Pin
;
Liu, Yu-Syun
;
Lin, Wen-Huei
;
資訊工程學系
;
Department of Computer Science
Commodity solid state drives (SSDs) have recently begun involving the adoption of powerful controllers for multichannel flash management at the page level. However, many of these models still use primitive garbage collection algorithms, because previous approaches are subject to poor scalability with high-capacity flash memory. This study presents Stable Greedy for garbage collection in page-mapping multichannel SSDs. Stable Greedy identifies page-accurate data hotness using block-level information, and jointly considers block space utilization and block stability for victim selection. Its design considers flash wear leveling for SSD lifetime enhancement at the block level as well as at the channel level. Stable Greedy runs at a constant time, and requires limited RAM space. The simulation results revealed that Stable Greedy outperformed previous methods considerably under various workloads and multichannel architectures. Stable Greedy was successfully implemented on the OpenSSD platform, and the actual performance measurements were consistent with the simulation results.
Exploiting Page Correlations for Write Buffering in Page-Mapping Multichannel SSDs
(2016)
Huang, Sheng-Min
;
Chang, Li-Pin
;
資訊工程學系
;
Department of Computer Science
Advanced solid-state disks (SSDs) have been equipped with page-mapping flash translation layers and multichannel architectures. The SSDs employ a RAM-based write buffer, which delays write requests for reducing write traffic, reorders requests for mitigating garbage-collection overhead, and produces parallel page writes for improving channel time utilization. This work presents a novel write buffer algorithm that exploits temporal and spatial correlations among buffer pages. The write-buffer groups temporally or spatially correlate buffer pages and then write the grouped buffer pages to the same flash block. In this way, when the correlated page data are updated in the future, flash blocks will receive bulk page invalidations and become good candidates for garbage collection. With multichannel architectures, the write buffer adaptively disperses read-most sequential data over channels for high page-level parallelism of sequential reads, while clustering write-most sequential data in the same channel for a reduced cost of garbage collection. We evaluated the proposed method and previously proposed buffer algorithms. Our method was shown to outperform the existing methods by up to 134%. We also implemented our buffer design on the OpenSSD platform; the time and space overheads of our design were reported to be very low.
Plugging Versus Logging: Adaptive Buffer Management for Hybrid-Mapping SSDs
(2015-03-01)
Chang, Li-Pin
;
Su, Yo-Chuan
;
Wu, I-Chen
;
資訊工程學系
;
Department of Computer Science
A promising technique to improve the write performance of solid-state disks (SSDs) is to use a disk write buffer. The goals of a write buffer is not only to reduce the write traffic to the flash chips but also to convert host write patterns into long and sequential write bursts. This study proposes a new buffer design consisting of a replacement policy and a write-back policy. The buffer monitors how the host workload stresses the flash translation layer upon garbage collection. This is used to dynamically adjust its replacement and write-back strategies for a good balance between write sequentiality and write randomness. When the garbage collection overhead is low, the write buffer favors high write sequentiality over low write randomness. When the flash translation layer observes a high overhead of garbage collection, the write buffer favors low write randomness over high write sequentiality. The proposed buffer design outperformed existing approaches by up to 20% under various workloads and flash translation algorithms, as will be shown in experiment results.
Reducing Asynchrony in Channel Garbage-Collection for Improving Internal Parallelism of Multichannel Solid-State Disks
(2014-01-01)
Chang, Li-Pin
;
Wen, Chen-Yi
;
交大名義發表
;
National Chiao Tung University
Solid-state disks use multichannel architectures to boost their data transfer rates. Because realistic disk workloads have numerous small write requests, modern flash-storage devices adopt a write buffer and a set of independent channels for better parallelism in serving small write requests. When a channel is undergoing garbage collection, it stops responding to inbound write traffic and accumulates page data in the write buffer. This results in contention for buffer space and creates idle periods in channels. This study presents a channel-management strategy, called garbage-collection advancing, which allows early start of garbage collection in channels for increasing the overlap among channel activities of garbage collection and restoring the balance of buffer-space usage among channels. This study further introduces cycle filling, which is a version of garbage-collection advancing tailored for the operation model of flash planes. Experimental results show that the proposed methods greatly outperformed existing designs of multichannel systems in terms of response and throughput. We also successfully implemented the proposed methods in a real solid-state disk and proved their feasibility in real hardware.
An Adaptive, Low-Cost Wear-Leveling Algorithm for Multichannel Solid-State Disks
(2013-12-01)
Chang, Li-Pin
;
Chou, Tung-Yang
;
Huang, Li-Chun
;
資訊工程學系
;
Department of Computer Science
Multilevel flash memory cells double or even triple storage density, producing affordable solid-state disks for end users. As flash memory endures only limited program-erase cycles, solid-state disks employ wear-leveling methods to prevent any portions of flash memory from being retired prematurely. Modern solid-state disks must consider wear evenness at both block and channel levels. This study first presents a block-level wear-leveling method whose design has two new ideas. First, the proposed method reuses the intelligence available in flash-translation layers so it does not require any new data structures. Second, it adaptively tunes the threshold of block-level wear leveling according to the runtime write pattern. This study further introduces a new channel-level wear-leveling strategy, because block-level wear leveling is confined to a channel, but realistic workloads do not evenly write all channels. The proposed method swaps logical blocks among channels for achieving an eventually-even state of channel lifetimes. A series of trace-driven simulations show that our wear-leveling method outperforms existing approaches in terms of wear evenness and overhead reduction.
A Low-Cost Wear-Leveling Algorithm for Block-Mapping Solid-State Disks
(2011)
Chang, Li-Pin
;
Huang, Li-Chun
;
資訊工程學系
;
Department of Computer Science
Multilevel flash memory cells double or even triple storage density, producing affordable solid-state disks for end users. However, flash lifetime is becoming a critical issue in the popularity of solid-state disks. Wear-leveling methods can prevent flash-storage devices from prematurely retiring any portions of flash memory. The two practical challenges of wear-leveling design are implementation cost and tuning complexity. This study proposes a new wear-leveling design that features both simplicity and adaptiveness. This design requires no new data structures, but utilizes the intelligence available in sector-translating algorithms. Using an on-line tuning method, this design adaptively tunes itself to reach good balance between wear evenness and overhead. A series of trace-driven simulations show that the proposed design outperforms a competitive existing design in terms of wear evenness and overhead reduction. This study also presents a prototype that proves the feasibility of this wear-leveling design in real solid-state disks.

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