Reducing Database Locking Contention Through Multi-version Concurrency

摘要

In multi-version databases, updates and deletions of records by transactions require appending a new record to tables rather than performing in-place updates. This mechanism incurs non-negligible performance overhead in the presence of multiple indexes on a table, where changes need to be propagated to all indexes. Additionally, an uncommitted record update will block other active transactions from using the index to fetch the most recently committed values for the updated record. In general, in order to support snapshot isolation and/or multi-version concurrency, either each active transaction is forced to search a database temporary area (e.g., rollback segments) to fetch old values of desired records, or each transaction is forced to scan the entire table to find the older versions of the record in a multi-version database (in the absence of specialized temporal indexes). In this work, we describe a novel kV-lndirection structure to enable efficient (parallelizable) optimistic and pessimistic multi-version concurrency control by utilizing the old versions of records (at most two versions of each record) to provide direct access to the recent changes of records without the need of temporal indexes. As a result, our technique results in higher degree of concurrency by reducing the clashes between readers and writers of data and avoiding extended lock delays. We have a working prototype of our concurrency model and kV-lndirection structure in a commercial database and conducted an extensive evaluation to demonstrate the benefits of our multi-version concurrency control, and we obtained orders of magnitude speed up over the single-version concurrency control.