Azor: Using Two-Level Block Selection to Improve SSD-Based I/O Caches

Flash-based Solid State Drives (SSDs) exhibit potential for solving I/O bottlenecks by offering superior performance over hard disks for several workloads. In this paper, the authors design Azor, an SSD-based I/O cache that operates at the block-level and is transparent to existing applications, such as databases. Their design provides various choices for associativity, write policies and cache line size, while maintaining a high degree of I/O concurrency. Their main contribution is that they explore differentiation of HDD blocks according to their expected importance on system performance. They design and analyze a two-level block selection scheme that dynamically differentiates HDD blocks, and selectively places them in the limited space of the SSD cache.