A study of background cleaning and data allocation for multi-channel SSDs

Due to the idiosyncrasies of NAND flash memories, such as erase-before-write and bulk erase, SSDs employ an internal garbage collection process to recycle obsolete data to provide free spaces. To overlap the cleaning with I/O services in parallel units of multi-channel SSDs, a finer-granularity cleaning has been proposed, namely background cleaning. This method is not widely accepted for its reliability and wear-leveling problems. In this work, we study the background cleaning with different striping allocation policies, analyzing the SSD performance and write amplification with diverse cleaning units. Trace-driven simulations reveal that with certain striping allocation policies, finer-granularity cleaning can improve the SSD performance, as well as keep comparable cleaning efficiency, even without using the copy-back command.