Hybrid Storage with Disk Based Write Cache

Abstract

Recently, flash-memory-based solid state disks (SSDs) have been considered to be alternatives for traditional magnetic disks. However, it has not come true so far due to some limitations on SSDs, such as high latency of write operation and low reliability in case of unbalanced erasure. Therefore, a practical way is to integrate SSD and magnetic disk and then to obtain a better tradeoff between those two storage medium. In this paper, we investigate the issues of integrating SSD and disk in the storage layer of a database management system. In particular, we propose a new approach to using a magnetic disk as the write cache of an SSD, in which each data page is placed either in disk or in SSD. To find an optimal page placement scheme, we first propose a page migration model, which uses two grains, namely page and block (a set of pages), to perform the migration between SSD and disk. Based on this model, we develop an online approach to determining the optimal places of data pages. We conduct experiments on tailor-made traces to measure the performance of our hybrid storage approach. The results show that our approach ensures most read operations are performed on SSD and most write operations are focused on disk. Meanwhile, our hybrid approach has less runtime than the single-disk-based mechanism.