VDB: Virtualizing the On-Board Disk Write Cache

In the cloud environment, virtual machine monitor (VMM) enables multiple virtual machines (VM) running simultaneously to share the same physical resources, such as disk and its on-board disk write-cache. However, for the reasons of high reliability, some virtual machines need to adopt write-through policy for disk write-cache, which is a key component to improve system´s I/O performance. This causes serious write performance degradation for other data-intensive virtual machines which prefer a write-back cache. To satisfy the requirement of reliability and performance for different VMs, we adopt a disk write-through cache for the virtual machines with high reliability requirement and create a replaced write cache at the VMM layer for each virtual machine with high write performance demand. We also design and implement a prototype system called VDB within Xen. Our experimental results show that our system solves the coexistence problem of reliable virtual machines and I/O-bound virtual machines for the use of disk write-cache on the same virtualization platform. It can guarantee the reliability requirement and improve the write performance for different VMs.