Write Policies for Host-side Flash Caches

Host-side flash-based caching offers a promising new direction for optimizing access to networked storage. Current work has argued for using host-side flash primarily as a read cache and employing a write-through policy which provides the strictest consistency and durability guarantees. However, write-through requires synchronous updates over the network for every write. For write-mostly or write-intensive workloads, it significantly under-utilizes the high-performance flash cache layer. The write-back policy, on the other hand, better utilizes the cache for workloads with significant write I/O requirements. However, conventional write-back performs out-of-order eviction of data and unacceptably sacrifices data consistency at the network storage.

We develop and evaluate two consistent write-back caching policies, ordered and journaled, that are designed to perform increasingly better than write-through. These policies enable new trade-off points across performance, data consistency, and data staleness dimensions. Using benchmark workloads such as PostMark, TPC-C, Filebench, and YCSB we evaluate the new write policies we propose alongside conventional write-through and write-back. We find that ordered write-back performs better than write-through. Additionally, we find that journaled write-back can trade-off staleness for performance, approaching, and in some cases, exceeding conventional write-back performance. Finally, a variant of journaled write-back that utilizes consistency hints from the application can provide straight forward application-level storage consistency, a stricter form of consistency than the transactional consistency provided by write-through.