Block Mason.

Hardware virtualization gives administrators the flexibility to rapidly create, destroy and relocate virtual machines across physical hosts. Unfortunately, the storage systems upon which these systems depend are not nearly as agile. To facilitate the rapid, safe development of block devices that can meet the needs of virtual machines, we present the Block Mason virtual block device framework. Although the block device interface is simple and intuitive, block devices themselves must generally be implemented in the operating system kernel, an environment which is neither simple nor portable. Block Mason allows users to build small, reusable block processing elements in user space, and to connect them together into powerful composite modules using a simple declarative graph language. Although the environment emphasizes simplicity for developers and end users, it includes built-in support for powerful operations like live reconfiguration and dependency tracking.