Semi-mirror : Performance Improvement of Mirroring for Heterogeneous Storage Environments

Abstract

A Solid State Drive (SSD) is a storage device that has the advantages of high speed and low power consumption, but that is more expensive than the Hard Disk Drive (HDD). A recently developed high-performance storage server combines the features of an SSD and an HDD, containing a large HDD and a smaller SSD cache to improve system performance of frequently accessed files. However, existing mirroring techniques such as RAID-1 between an SSD and a HDD are not suitable for write-intensive workloads because of the slow HDDs random write performance. Addressing this issue, this paper presents a new mirroring technique, named semi-mirror, built on top of Linuxs device mapper architecture. It has three characteristics. First, it exploits the HDD bandwidth in append-only fashion. Second, it offers the same availability and reliability as existing mirroring techniques. Lastly, in the case of power or mechanical failure of the primary device, semi-mirror offers a recovery process, making a copy from the mirror device (HDD) to the primary device (SSD). Semi-mirror shows 26 times higher performance than dm-raid1 in case of random write in IOzone, and it leads to 5-6 times performance improvement in TPC-C.