CachedGC: Cache-Assisted Garbage Collection in Modern Solid State Drives

Solid State Drives (SSDs) have many attractive features, and they are well-positioned to replace traditional HDDs in cloud and enterprise storages. However, some specific features they have such as erase-before-write and limited erase count make this transition more challenging. To tackle the issues brought by these features, Flash Translation Layer (FTL) leverages functionalities such as Garbage Collection (GC) to provide free space by erasing the blocks and cleaning the stale data. This process occupies SSD resources and causes extra waiting time for normal IO requests, and ultimately increases response time. Cloud workloads include many applications with small random write access patterns which cause frequent GC invocations. Flash occupation at the time of GC causes inconsistent performance and increases tail latency which may in turn violate SLA in these applications. In this paper, we modify the conventional GC process and propose a Cache-assisted Garbage Collection (CachedGC). CachedGC postpones writing back valid pages during the GC, erases the block at the first possible time, and hence resumes normal SSD operations sooner. CachedGC reduces request response time and tail latency by reducing the effective latency of GC. Our experimental results using a variety of workloads show that CachedGC reduces read tail latency by 19% and improves write IOPS by as much as 27%.