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Corporate InformationResearch & Development

November 29, 2013

Report from Presenter

The Second Asian Conference on Information Systems (ACIS 2013) was held in Phuket, Kingdom of Thailand during October 31st to November 2nd 2013. This conference widely covered many topics about information systems such as data mining, sensor systems, and artificial intelligence.

Yokohama Research Laboratory (YRL), Hitachi Ltd. presented two papers. The following introduces one of our papers titled "Evaluation of SSD Tier Method and SSD Cache Method in Tiered Storage System."

With recent improvements in information technology, the amount of access workload to information systems is increasing. In general, input and output (I/O) activities have a locality, and the number of I/Os for each area is different. Therefore, if frequently accessed areas are stored on SSDs (Solid State Drives) that have high performance and are expensive, and rarely accessed areas are stored on HDDs (Hard Disk Drives) that have low performance and are inexpensive, I/O performance will increase and system cost will be reduced. There are two methods to utilize SSD on the storage having both SSD and HDD.

  1. SSD Tier Method: Method Dividing data and locating the data to SSD or HDD
  2. SSD Cache Method: Method locating all data to HDD and coping partial data to SSD

The SSD tier method improves I/O performance by allocating hot areas to SSD. However, it takes 1 hour to 24 hours to improve the I/O performance because this method optimizes data location every 1 hour to 24 hours when the hot areas move. Since the SSD cache method caches accessed area to SSD immediately, it improves the second and subsequence accesses to the area. Performance does not improve when cached data is frequently purged form SSD cache before data is accessed.

Fig. 1 Problem with existing method

Fig. 2 Proposed method

We considered that utilizing both methods simultaneously addresses the problems above. We assumed that appropriate number of SSDs for tier and appropriate number of SSDs for cache may differ on the basis of the number of I/O, read-write ratio, and I/O locality. In this research, we evaluated the effects of these methods by I/O simulation and clarified conditions when these methods are effective.

The simulation result shows that the SSD tier method improves the storage I/O performance in write-intensive and low I/O locality workload with small SSD capacity. The SSD cache method is effective in read-intensive and high I/O locality workload with large SSD capacity. Utilizing both methods simultaneously with write-intensive workload is the most effective. The storage I/O response time is reduced by up to 28% utilizing both methods simultaneously compared to using the SSD tier method or the SSD cache method separately.

(By HAYASHI Shinichi)

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