Isotope: Transactional Isolation for Block Storage

Existing storage stacks are top-heavy and expect little from block storage. As a result, new high-level storage abstractions - and new designs for existing abstractions - are difficult to realize, requiring developers to implement from scratch complex functionality such as failure atomicity and fine-grained concurrency control. In this paper, we argue that pushing transactional isolation into the block store (in addition to atomicity and durability) is both viable and broadly useful, resulting in simpler high-level storage systems that provide strong semantics without sacrificing performance. We present Isotope, a new block store that supports ACID transactions over block reads and writes. Internally, Isotope uses a new multiversion concurrency control protocol that exploits fine-grained, sub-block parallelism in workloads and offers both strict serializability and snapshot isolation guarantees. We implemented several high-level storage systems over Isotope, including two key-value stores that implement the LevelDB API over a hashtable and B-tree, respectively, and a POSIX filesystem. We show that Isotope's block-level transactions enable systems that are simple (100s of lines of code), robust (i.e., providing ACID guarantees), and fast (e.g., 415 MB/s for random file writes). We also show that these systems can be composed using Isotope, providing applications with transactions across different high-level constructs such as files, directories and key-value pairs.