The recent decade witnesses the prevalence of hybrid storage systems that offer storage based on diverse storage devices to achieve both cost savings and performance improvements. However, there have been little of studies focusing on the challenging scenario of providing hybrid block storage for Virtual Machines (VMs), which is subject to several stringent requirements such as availability and scalability. To bridge this gap, we designed and implemented a multi-tiered storage system called MOBBS which provides hybrid block storage for VMs on top of object-based storage infrastructure. MOBBS achieves efficient utilization of different storage devices by striping data into partitions and intelligently determining appropriate data placement by monitoring and analyzing real-time workloads. Moreover, leveraging the benefits of object-based storage, several techniques are designed to promise storage provisioning for VMs with high availability and scalability. A Ceph implementation of MOBBS is evaluated using both block and file system workloads. The results demonstrate the effectiveness and efficiency of MOBBS in utilization of different storage devices.