Kvell+: Snapshot Isolation without Snapshots

Snapshot Isolation (SI) enables online analytical processing (OLAP) queries to observe a snapshot of the data at the time the query is issued, despite concurrent updates by online transactional processing (OLTP) transactions. The conventional implementation of SI creates a new version of a data item when it is updated, rather than overwriting the old version. Versions are garbage collected when they can no longer be read by any OLAP query. Frequent updates during long-running OLAP queries therefore create significant space amplification, and garbage collection can give rise to latency spikes for OLTP transactions. These problems are exacerbated on modern low-latency drives that can persist millions of updates per second.

We observe that analytic queries often consist in large part of commutative processing of data items resulting from range scans in which each item in the range is read exactly once. We introduce Online Commutative Processing (OLCP), a new model for processing analytical queries, that takes advantage of this observation. Under OLCP, analytical queries observe the same snapshot of the data as they would under conventional SI, but space amplification and garbage collection costs are largely and oftentimes nearly entirely avoided. When an item in such a range is updated, the old version of the item is propagated to the OLCP queries that might need it instead of being kept in the store.

We demonstrate OLCP's expressiveness by showing how to formulate, among others, the TPC-H benchmark queries in OLCP. We implement OLCP in KVell+, an extension of KVell, a key-value store for NVMe SSDs. Using YCSB-T, TPC-CH and production workloads from Nutanix, we run a wide range of analytics queries concurrently with writeintensive transactions. We show that OLCP incurs little or no space amplification or garbage collection overhead. As a surprising by-product we also show that OLCP speeds up analytical queries compared to SI.