Performance Evaluation of Dynamic Page Allocation Strategies in SSDs

Solid-state drives (SSDs) with tens of NAND flash chips and highly parallel architectures are widely used in enterprise and client storage systems. As any write operation in NAND flash is preceded by a slow erase operation, an out-of-place update mechanism is used to distribute writes through SSD storage space to postpone erase operations as far as possible. SSD controllers use a mapping table along with a specific allocation strategy to map logical host addresses to physical page addresses within storage space. The allocation strategy is further responsible for accelerating I/O operations through better striping of physical addresses over SSD parallel resources. Proposals already exist for using static logical-to-physical address mapping that does not balance the I/O traffic load within the SSD, and its efficiency highly depends on access patterns. A more balanced distribution of I/O operations is to alternate resource allocation in a round-robin manner irrespective of logical addresses. The number of resources that can be dynamically allocated in this fashion is defined as the degree of freedom, and to the best of our knowledge, there has been no research thus far to show what happens if different degrees of freedom are used in allocation strategy. This article explores the possibility of using dynamic resource allocation and identifies key design opportunities that it presents to improve SSD performance. Specifically, using steady-state analysis of SSDs, we show that dynamism helps to mitigate performance and endurance overheads of garbage collection. Our steady-state experiments indicate that midrange/high-end SSDs with dynamic allocation can provide I/O operations per second (IOPS) improvement of up to 3.3x/9.6x, response time improvement of up to 56%/32%, and about 88%/96% average reduction in the standard deviation of erase counts of NAND flash blocks.