Virtualization technology enables multiple virtual machines(VMs) to run on a physical mechanism. Due to the excessive data-intensive workloads of VMs, the system I/O performance of cloud platforms deteriorates. To improve the system I/O performance, one key technique is to exploit the spatial locality of data stored on the back-end disks. However, identifying data spatial locality on a cloud platform becomes challenging due to the transparency feature of virtualization. Therefore, to resolve the problem of poor disk I/O, this paper proposes an inter-VMs locality packing design to increase the number of sequential I/O accesses on back-end disks so as to improve disk I/O efficiency through exploiting the spatial locality of virtual machine images. Moreover, the proposed deign does not compromise the transparency of virtualization technique. A simulator architecture is also proposed to assess the performance of the proposed inter-VMs locality packing design.