A Transactional Approach to Redundant Disk Array Implementation (CMU-CS-97-141)

Redundant disk arrays are a popular method of improving the dependability and performance of disk storage and an ever-increasing number of array architectures are being proposed to balance cost, performance, and dependability. Despite their differences, there is a great deal of commonality between these architectures; unfortunately, it appears that current implementations are not able to effectively exploit this commonality due to their ad hoc approach to error recovery. Such techniques rely upon a case-by-case analysis of errors, a manual process that is tedious and prone to mistakes. For each distinct error scenario, a unique procedure is implemented to remove the effects of the error and complete the affected operation. Unfortunately, this form of recovery is not easily extended because the analysis must be repeated as new array operations and architectures are introduced. Transaction-processing systems utilize logging techniques to mechanize the process of recovering from errors. However, the expense of guaranteeing that all operations can be undone from any point in their execution is too expensive to satisfy the performance and resource requirements of redundant disk arrays. This dissertation describes a novel programming abstraction and execution mechanism based upon transactions that simplifies implementation. Disk array algorithms are modeled as directed acyclic graphs: the nodes are actions such as "XOR" and the arcs represent data and control dependencies between them. Using this abstraction, we implemented eight array architectures in RAIDframe, a framework for prototyping disk arrays. Code reuse was consistently above 90%. The additional layers of abstraction did not affect the response time and throughput characteristics of RAIDframe; however, RAIDframe consumes 60% more CPU cycles than a hand-crafted non-redundant implementation. RAIDframe employs roll-away error recovery, a novel scheme for mechanizing the execution of disk array algorithms without requiring that all actions be undoable. A barrier is inserted into each algorithm: failures prior to the barrier result in rollback, relying upon undo information. Once the barrier is crossed, the algorithm rolls forward to completion, and undo records are unnecessary. Experiments revealed this approach to have identical performance to that of non-logging schemes.