III: Small: Multi-Version Concurrency Control on Modern Hardware

就更新数据的方式而言，数据库系统通常分为两类：要么用新值覆盖旧值，要么存储（并使其可访问）旧版本和新版本的数据。大多数早期的数据库系统属于第一类（覆盖数据），因为存储昂贵且有限。然而，随着合规性、审计、报告、备份和灾难恢复用例越来越依赖于读取数据库快照和稍后更新的数据，数据库系统以近线访问模式维护记录值的最新版本的能力变得越来越重要。此外，维护记录值的多个版本可能使数据库能够提高处理事务请求的吞吐量 - 同一记录的读取和写入可能会同时发生，而不是延迟事务以避免不一致的读取。因此，数据库系统架构的趋势是第二类：保留数据值的多个版本。现有的多版本数据库系统在处理当今存在的合规性、审计、报告、备份和灾难恢复用例方面已经做了足够的工作。然而，在许多情况下，它们无法针对快照隔离异常提供保护，这种异常会违反数据库中数据的正确性，并可能导致在数据库系统之上编写的应用程序代码出现错误。少数现有系统可以避免这些异常，但这样做的代价是吞吐量和数据库性能的显着损失。此外，大多数现有的多版本数据库系统都是为磁盘存储而设计的，并且很难在主内存工作负载下表现良好。我们正在构建一个完全重新设计的多版本数据库系统，专为主内存部署而设计。它的架构旨在实现极高的吞吐量，同时避免以前系统中存在的影响应用程序开发人员和数据库用户的基本写入偏差异常。我们正在将数据库可恢复性和事务斩波的新技术集成到多版本数据库系统架构中，以便将事务吞吐量提高至少一个数量级。我们还在研究连续快照复制的技术，并为具有有限陈旧性的地理分布式副本提供一致的读取查询。此外，我们正在进行广泛的实验研究，这些研究既促进了我们系统的设计，也促进了多版本数据库系统与单版本系统的一般使用。

Database systems generally fall into two categories with respect to how they update data: either they overwrite the old value with the new value, or they store (and make accessible) both the old version and new version of the data. Most early database systems fall in the first category (overwriting data) because storage was expensive and limited. However, as compliance, auditing, reporting, back-up, and disaster recovery use-cases increasingly rely on reading database snapshots and data that may have been later updated, the ability of a database system to maintain recent versions of record values in near-line access modes has become increasingly important. Furthermore, maintaining multiple versions of record values potentially enables the database to improve its throughput of handling transaction requests - reads and writes of the same record can potentially occur concurrently instead of delaying transactions to avoid inconsistent reads. Therefore, the trend in database system architecture is towards the second category: keeping around multiple versions of data values. Existing multi-versioned database systems have done an adequate job in handling the compliance, auditing, reporting, back-up, and disaster recovery use-cases that exist today. However, in many cases they provide no protection against snapshot isolation anomalies that violates the correctness of the data in the database and can result in bugs in application code written on top of the database system. The few existing systems that avoid these anomalies only do so at significant costs to throughput and database performance. Furthermore, most existing multi-versioned database systemsare designed for disk-storage, and struggle to perform well under main-memory workloads.We are building a completely redesigned multi-versioned database system, designed for main-memory deployments. It is architected to achieve extremely high throughput, while avoiding the fundamental write skew anomalies that have existed in previous systems affecting both application developers and database users. We are integrating novel techniques for database recoverability and transaction chopping into the multi-versioned database system architecture in order to improve transaction throughput by at least an order of magnitude. We are also investigating techniques for continuous snapshot replication and serving consistent read queries from geo-distributed replicas with bounded staleness. Furthermore, we are performing extensive experimental studies that both motivate the design of our system, and also motivate the use of multi-versioned database systems in general vs. single-versioned systems.