• International journal of advanced smart convergence
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• v.10 no.1
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• pp.142-150
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• 2021

Open-channel SSD is a new type of Solid-State Disk (SSD) that improves the garbage collection overhead and write amplification due to physical constraints of NAND flash memory by exposing the internal structure of the SSD to the host. However, the host-level Flash Translation Layer (FTL) provided for open-channel SSDs in the current Linux kernel consumes host memory excessively because it use page-level mapping table to translate logical address to physical address. Therefore, in this paper, we implemente a selective mapping table loading scheme that loads only a currently required part of the mapping table to the mapping table cache from SSD instead of entire mapping table. In addition, to increase the hit ratio of the mapping table cache, filesystem information and mapping table access history are utilized for cache replacement policy. The proposed scheme is implemented in the host-level FTL of the Linux kernel and evaluated using open-channel SSD emulator. According to the evaluation results, we can achieve 80% of I/O performance using the only 32% of memory usage compared to the previous host-level FTL.

• Journal of Advanced Navigation Technology
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• v.13 no.5
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• pp.684-690
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• 2009

In this paper, we propose the acceleration method of the DDR-SSD RAID level 5. The DDR-SSD is the storage device of the Next Generation Storage(NGS) system. The DDR-SSD has different characteristics with HDD and Flash SSD. That's why the DDR-SSD RAID level 5 does not provide the best performance when the normal acceleration method is used. In this paper, to accelerate the DDR-SSD RAID level 5 operation, we propose the parity cache and the architecture of the parity cell. The parity cache stores only parity blocks. This acceleration method proposed in this paper reduce the number of the disk access and the overhead of parity operations.

• Journal of KIISE:Computing Practices and Letters
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• v.16 no.3
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• pp.371-375
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• 2010

SSD is a storage medium based on NAND Flash memory. Because of its short latency, low power consumption, and resistance to shock, it's not only used in PC but also in server computers. Most SSDs use FTL to overcome the erase-before-overwrite characteristic of NAND flash. There are several types of FTL, but page mapped FTL shows better performance than others. But its usefulness is limited because of its large memory footprint for the mapping table. For example, 64MB memory space is required only for the mapping table for a 64GB MLC SSD. In this paper, we propose a novel caching scheme for the mapping table. By using the mapping-table-meta-data we construct a fully associative cache, and translate the address within O(1) time. The simulation results show more than 80 hit ratio with 32KB cache and 90% with 512KB cache. The overall memory footprint was only 1.9% of 64MB. The time overhead of cache miss was measured lower than 2% for most workload.

• Proceedings of the Korean Society of Computer Information Conference
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• 2014.07a
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• pp.343-345
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• 2014

플래시 메모리의 인기가 증가하면서 스토리지 시스템의 변화를 가져왔다. 플래시 메모리 기반의 SSD(Solid State Disk)는 기존의 HDD(Hard Disk Drive)를 대체할 매체로 주목을 받고 있으며 HDD에 비해 훨씬 더 높은 대역폭, 랜덤 접근 성능 및 충격에 강한 장점들을 갖는다. 그러나 플래시 메모리는 HDD와 달리 덮어쓰기(In-Place update)가 불가능 하기 때문에, 데이터를 업데이트 하기 위해서는 해당영역을 지운 후 업데이트를 해야 하는 단점이 있다. 본 논문에서는 캐시 안에서의 거의 접근하지 않을 블락과 캐시로 들어가는 것을 막기 위한 기법을 제시한다. 이것은 캐시의 오염을 막고 더 오랜 기간 동안 캐시 안에서 인기 있는 블락 들을 유지하고 높은 히트율로 연결될 것이다. 또한 캐시 교체의 수를 줄임으로써 SSD의 쓰기를 감소할 것이고 그 결과 성능 뿐만 아니라 SSD의 수명도 연장 에도 도움이 될 것이다.

• Proceedings of the Korean Society of Computer Information Conference
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• 2014.07a
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• pp.235-236
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• 2014

플래시 메모리 기술이 발전함에 따라 낸드 플래시 기반의 SSD가 상용화 되면서 I/O시간을 줄이기 위한 연구들이 진행되고 있다. 이에 본 논문에서는 기존의 메인 메모리와 저장장치 사이에 확장 버퍼 캐시로써 SSD를 사용하고 메인 메모리에서 방출 된 페이지들을 구분하여 같은 성향의 페이지들을 블록화 하는 모델을 제안한다. 이러한 모델을 통하여 블록 단위로 사용되는 SSD를 효율적으로 이용하여 읽기 및 쓰기 성능을 높이고 I/O에 해당하는 시간들을 줄임으로써 전체적인 성능 향상을 증명하였다.

• Journal of KIISE
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• v.42 no.8
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• pp.954-960
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• 2015

It is important to improve the performance of storage in the home cloud environment within the virtualized IoT gateway since the performance of applications deeply depends on storage. Though SSD caching is applied in order to improve the storage, it is only used for read-cache due to the limitations of SSD such as poor write performance and small write endurance. However, it is important to improve performance of the write operation in the home cloud server, in order to improve the end-user experience. This paper propose a novel SSD caching which considers write-data as well as read-data. We validate the enhancement in the performance of random-write by transforming it to the sequential patterns.

• KIPS Transactions on Computer and Communication Systems
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• v.8 no.7
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• pp.159-166
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• 2019

Unlike modern Solid-State Drives with DRAM, DRAM-less SSDs do not have DRAM because they are cheap and consume less power. Obviously, they have performance degradation problem due to lack of DRAM in the controller and this problem can be alleviated by utilizing host memory buffer(HMB) feature of NVMe, which allows SSDs to utilize the DRAM of host. In this paper, we show that commercial DRAM-less SSDs surely exhibit lower I/O performance than other SSDs with DRAM, but they can be improved by utilizing the HMB feature. Through various experiments and analysis, we also show that DRAM-less SSDs mainly exploit the DRAM of host as mapping table cache rather than read cache or write buffer to improve I/O performance.

• Journal of KIISE:Databases
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• v.37 no.6
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• pp.308-314
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• 2010

As the price of flash memory continues to drop and the technology of flash SSD controller innovates, high performance flash SSDs with affordable prices flourish in the storage market. Nevertheless, it is hard to expect that flash SSDs will replace harddisks completely as database storage. Instead, the approach to use flash SSD as a cache for harddisks would be more practical, and, in fact, several hybrid storage architectures for flash memory and harddisk have been suggested in the literature. In this paper, we propose a new approach to use flash SSD as an extended buffer for main buffer in database systems, which stores the pages replaced out from main buffer and returns the pages which are re-referenced in the upper buffer layer, improving the system performance drastically. In contrast to the existing approaches to use flash SSD as a cache in the lower storage layer, our approach, which uses flash SSD as an extended buffer in the upper host, can provide fast random read speed for the warm pages which are being replaced out from the limited main buffer. In fact, for all the pages which are missing from the main buffer in a real TPC-C trace, the hit ratio in the extended buffer could be more than 60%, and this supports our conjecture that our simple extended buffer approach could be very effective as a cache. In terms of performance/price, our extended buffer architecture outperforms two other alternative approaches with the same cost, 1) large main buffer and 2) more harddisks.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.4
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• pp.117-123
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• 2013

In the recent cloud storage environment, the amount of SSD (Solid-State Drive) replacing with the traditional hard disk drive is increasing. Management of SSD for its space efficiency has become important since SSD provides fast IO performance due to no mechanical movement whereas it has wearable characteristics and does not provide in place update. In order to manage space efficiency of SSD, data de-duplication technique is frequently used. However, this technique occurs much overhead because it consists of data chunking, hasing and hash matching operations. In this paper, we propose new data de-duplication method using PRAM cache. The proposed method uses hierarchical hash tables and LRU(Least Recently Used) for data replacement in PRAM. First hash table in DRAM is used to store hash values of data cached in the PRAM and second hash table in PRAM is used to store hash values of data in SSD storage. The method also enhance data reliability against power failure by maintaining backup of first hash table into PRAM. Experimental results show that average writing frequency and operation time of the proposed method are 44.2% and 38.8% less than those of existing data de-depulication method, respectively, when three workloads are used.

• Journal of KIISE:Computing Practices and Letters
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• v.16 no.4
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• pp.462-466
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• 2010

A new buffer cache management scheme that aims at reducing power consumption in mixed HDD and NAND flash memory storage systems is presented. The proposed scheme reduces power consumption by considering different energy-consumption rate of storage devices, I/O operation type (read or write), and reference potential of cached blocks in terms of both recency and frequency. Simulation shows that the proposed scheme reduces power consumption by 18.0% on average and up to 58.9%.