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

数据库系统在更新数据的方式上通常分为两类：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都属于第一类（存储数据），因为存储空间昂贵且有限。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阅读数据库快照和可能后来更新的数据，数据库系统在近线访问模式中维护记录值的最近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其处理事务请求的吞吐量-对同一记录的读取和写入可能并发发生，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朝着第二类：保持数据值的多个版本。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得很好。但是，在许多情况下，它们无法提供针对快照隔离异常的保护，快照隔离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中出现错误。现有的少数几个系统避免了这些异常，但这样做会对吞吐量和数据库性能造成很大的损失。此外，大多数现有的多版本数据库系统都是为磁盘存储设计的，并且在主存负载下很难表现良好。我们正在构建一个完全重新设计的多版本数据库系统，专为主存部署而设计。它的架构实现了极高的吞吐量，同时避免了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斜异常。我们正在将数据库可恢复性和事务切割的新技术集成到多版本数据库系统架构中，以提高事务吞吐量至少一个数量级。我们还在研究连续快照复制技术，并从具有有限陈旧性的地理分布副本中提供一致的读取查询。此外，我们正在进行广泛的实验研究，既激励我们的系统的设计，也激励多版本数据库系统在一般与单版本系统的使用。

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.