MapperX: Adaptive Metadata Maintenance for Fast Crash Recovery of DM-Cache Based Hybrid Storage Devices

DM-cache is a component of the device mapper of Linux kernel, which has been widely used to map SSDs and HDDs onto higher-level virtual block devices that take fast SSDs as a cache for slow HDDs to achieve high I/O performance at low monetary cost. While enjoying the benefit of persistent caching where SSDs accelerate normal I/O without affecting durability, the current design of DM-cache suffers from long crash recovery times (at the scale of hours) and low availability. This is because its metadata of dirty bits has to be asynchronously persisted for high I/O performance, which consequently causes all cached data on SSDs to be assumed dirty and to be recovered after the system is restarted.

This paper presents MapperX, a novel extension to DMcache that uses an on-disk adaptive bit-tree (ABT) to synchronously maintain the metadata of dirty bits in a hierarchical manner. Leveraging spatial locality of block writes, Map-perX achieves controlled metadata persistence overhead with fast crash recovery by adaptively adding/deleting leaves in the ABT where different levels represent the states of blocks with different granularity. We have implemented MapperX for Linux DM-cache module. Experimental results show that the MapperX based hybrid storage device outperforms the original DM-cache based hybrid device by orders of magnitude in crash recovery times while only introducing negligible metadata persistence overhead.