Hyperscale Resilient Buffer Pool Extension in Azure SQL Database

Azure SQL DB offers disaggregated storage architecture called Hyperscale. While this architecture provides storage scale out, it comes at the cost of performance of I/O from remote storage. In-memory caches on the Compute nodes are small and lost on process restarts. This paper introduces Resilient Buffer Pool Extension (RBPEX) which is a persistent cache present on both compute and storage nodes. These caches significantly improve performance of I/O from Compute while at the same time maintaining correctness. This paper presents the architecture of RBPEX and how it stores the most relevant pages in an efficient and correct way. It uses innovative techniques like statistics, selective caching and reduction of small writes to improve performance in Hyperscale. It also presents details about handling multiple versions of pages from storage to maintain correctness.