SSD-HDD-Hybrid Virtual Disk in Consolidated Environments ★ Heeseung Jo, Youngjin Kwon, Hwanju Kim, Euiseong Seo, Joonwon Lee, and Seungryoul Maeng Korea Advanced Institute of Science and Technology (KAIST), 335 Gwahangno, Yuseong-gu, Daejeon, Korea Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon-ri, Eonyang-eup, Ulju-gun, Ulsan, Korea Sungkyunkwan university, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, Korea {heesn,yjkwon,hjukim,maeng}@camars.kaist.ac.kr, euiseong@unist.ac.kr, joonwon@skku.edu Abstract. With the prevalence of multi-core processors and cloud com- puting, the server consolidation using virtualization has increasingly ex- panded its territory, and the degree of consolidation has also become higher. As a large number of virtual machines individually require their own disks, the storage capacity of a data center could be exceeded. To ad- dress this problem, copy-on-write storage systems allow virtual machines to initially share a template disk image. This paper proposes a hybrid copy-on-write storage system that combines solid-state disks and hard disk drives for consolidated environments. In order to take advantage of both devices, the proposed scheme places a read-only template disk im- age on a solid-state disk, while write operations are isolated to the hard disk drive. In this hybrid architecture, the disk I/O performance benefits from the fast read access of the solid-state disk, especially for random reads, precluding write operations from the degrading flash memory per- formance. We show that the hybrid virtual disk, in terms of performance and cost, is more effective than the pure copy-on-write disks for a highly consolidated system. Key words: Consolidation, Virtual machine (VM), Copy-on-write (CoW), Hybrid storage. 1 Introduction Virtualization enables multiple operating systems to run on a single physical machine, and server consolidation systems using virtualization have expanded ★ This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. 2009-0080381) and the MKE (Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) Support program supervised by the IITA (Institute of Information Technology Advancement) (IITA-2009-C1090-0902-0020).