SSD (solid state drive) has been widely deployed due to its high performance. But, its cost and reliability have not met the demand of large-scale storage systems. RAID (redundant arrays of inexpensive disks) is a conventional scheme to enhance reliability. However, existing RAID schemes are not effective for SSDs anymore. Instead, we propose a hybrid array consisting of SSDs and HDDs (hard disk drive): SSDs are used for responding to I/O requests, and HDDs supply backup for SSDs. The hybrid array guarantees both performance and reliability at a reasonable cost. However, HDDs lose to SSDs in term of both latency and bandwidth. Therefore, we employ a nonvolatile memory to bridge the latency gap, and take other two measures to improve HDD's bandwidth. Firstly, workloads within an HDD are reconfigured to be more sequential. Secondly, multiple HDDs collaborate with each other to supply much higher aggregate bandwidth. By these ways, HDDs can replicate SSDs' data in time. We implement a prototype to evaluate the proposed array. First of all, we demonstrate that the hybrid array is feasible. Then, we compare the hybrid array with other solutions. Experimental results show that the hybrid array covers both performance and reliability, and is also cost-effective.%固态盘(solid state drive,SSD)因为其优越的性能已被大量部署于当前的存储系统中.但是,由于寿命有限,SSD的可靠性受到广泛的质疑.磁盘阵列(redundant arrays of inexpensive disk,RAID)是一种传统的用来提高可靠性的手段,但并不适用于SSD.这项工作提出一种基于SSD和磁盘的混合存储系统,构建该系统的主要思想是SSD响应所有I/O请求,从而获得较高的性能；磁盘备份所有数据,从而保证系统的可靠性.但是,磁盘的I/O性能显著低于SSD,构建该系统的问题在于磁盘能否及时地备份SSD上的数据.为了解决这一问题,从两方面提出优化:在延迟方面,采用非易失主存弥补磁盘与SSD的延迟差距；在带宽方面,采用两种措施:1)在单块磁盘内部重组I/O请求,使磁盘尽可能的顺序读写；2)采用多块磁盘备份多块SSD,通过将一块SSD上的写请求分散到多块磁盘上,有效应对单块SSD上出现的突发写请求.通过原型系统实现表明,该混合系统是可行的:磁盘能够为SSD提供实时的数据备份；与其他系统相比,该混合系统取得较高的性价比.
展开▼
