MySQL 8.0复制新特性

导读

MySQL 8.0 复制功能有很大改进提升，并行复制性能与5.7相比可能提高数倍，是不是很期待？

翻译团队：知数堂藏经阁项目 - 琅琊阁

团队成员：琅琊阁-小剑伯、 琅琊阁-江b 、琅琊阁-简小鹿 

原文出处：https://severalnines.com/blog/what-s-new-mysql-replication-mysql-80

原文作者：Krzysztof Ksiazek

备注：发稿时，小编发现作者修改了原文，略尴尬哈

截止目前（2017年8月），MySQL 8.0 仍然是 beta 版本，复制功能有一些很棒的改进。最初，这些改进是为组复制（GR）开发的，但由于 GR 在底层使用常规复制，所以传统的 MySQL 复制也能由此获益。

我们这里提到的改进是存储在binlog中的依赖关系跟踪信息。MySQL 8.0 使用某种方法来存储那些受事务影响的行的相关信息（这些行被称为writeset），且它会比较来自不同事务中的writesets。这样就能识别出那些修改的数据行没有交集的事务，那么这些事务就可以在从库上被并行回放。与 MySQL 5.7 的实现相比，这也许能增加数倍的并行化程度。

要注意，从库上可能会出现与在主库上出现的不一样的数据视图（比如查询数据时默认的显示顺序和在主库上查询结果不同）。这是因为事务可能被按照与主库不同的顺序去回放。当然，这其实没有什么问题。目前在 MySQL 5.7 中实现的多线程复制也可能会导致这个问题，除非您明确地启用 slave-preserve-commit-order 参数。

MySQL 8.0, which as of now (August 2017) is still in beta state, brings some nice improvements to replication.Originally, it was developed for Group Replication (GR), but as GR uses regular replication under the hood, “normal” MySQL replication benefited from it.

The improvement we mentioned is dependency tracking information stored in the binary log. What happens is that MySQL 8.0 now has a way to store information about which rows were affected by a given transaction (so called writeset), and it compares writesets from different transactions. This makes it possible to identify those transactions which did not work on the same subset of rows and, therefore, these may be applied in parallel. This may allow to increase the parallelization level by several times compared to the implementation from MySQL 5.7. 

What you need to keep in mind is that, eventually, a slave will see a different view of the data, one that never appeared on the master. This is because transactions may be applied in a different order than on the master. This should not be a problem though.

The current implementation of multithreaded replication in MySQL 5.7 may also cause this issue unless you explicitly enable slave-preserve-commit-order.

为了控制这个新的行为（从库上数据回放顺序），可新增选项"binlog_transaction_dependency_tracking”，它可以取以下三个值：

• COMMIT_ORDER：默认值，它使用 MySQL 5.7 中可用的默认机制。

• WRITESET：它能实现更好的并行化，并且在主库的二进制日志中存储writeset数据。

• WRITE_SESSION：它确保事务在从库中按顺序执行，并且消除了从库中看到主库从未出现过的数据库状态的问题。它降低了并行化程度，但是仍然提供了比默认设置更好的吞吐量。

To control this new behavior, a variable binlog_transaction_dependency_tracking has been introduced. It can take three values:

COMMIT_ORDER: this is the default one, it uses the default mechanism available in MySQL 5.7.

WRITESET: It enables better parallelization and the master starts to store writeset data in binary log.

WRITESET_SESSION: This ensures that transactions will be executed on the slave in order and the issue with a slave that sees a state of database which never was seen on the master is eliminated. It reduces parallelization but it still can provide better throughput than the default settings.

基准测试

7月份，在 mysqlhighavailability.com 网站上，Vitor Oliveira 写了一篇文章分析了他对新模式进行性能测试的情况。他使用 MySQL 中性能最好的情况 —— 无持久性设置，展示了新旧模式之间的区别。

我们决定使用相同的方法进行测试，但同时设置一个更加真实的配置，启用 log_slave_updates 参数使得slave上也记录binlog。关于持久性设置，除了将innodb_flush_log_at_trx_commit 设置为 2 ，其他均保留默认值（所以，sync_binlog=1 —— 这是 MySQL 8.0 中的新默认值，启用 doublewrite buffer，启用 InnoDB checksums ，等等）。

In July, on mysqlhighavailability.com, Vitor Oliveira wrote a post where he tried to measure the performance of new modes.He used the best case scenario - no durability whatsoever, to showcase the difference between old and new modes.

We decided to use the same approach, this time in a more real-world setup: binary log enabled withlog_slave_updates. Durability settings were left to default (so, sync_binlog=1 - that’s new default in MySQL 8.0, doublewrite buffer enabled, InnoDB checksums enabled etc.) Only exception in durability wasinnodb_flush_log_at_trx_commit set to 2.

我们使用 m4.2xl 实例，32G，8核（所以参数 slave_parallel_workers 也设置为 8）。我们同样使用sysbench，oltp_read_write.lua 脚本。32 张表中的 1600 万行数据，存储在 1000GB gp2 卷（IOPS指标为3000）上。我们测试了 1、2、4、8、16、32 个并发连接下的所有模式，过程如下：关闭从库，执行 10 万个事务，启动从库并计算从库消除延迟所需的时间。

We used m4.2xl instances, 32G, 8 cores (so slave_parallel_workers was set to 8). We also used sysbench,oltp_read_write.lua script. 16 million rows in 32 tables were stored on 1000GB gp2 volume (that’s 3000 IOPS). We tested the performance of all of the modes for 1, 2, 4, 8, 16 and 32 concurrent sysbench connections. Process was as follows: stop slave, execute 100k transactions, start slave and calculate how long it takes to clear the slave lag.

首先，我们其实并不清楚使用单线程的 sysbench 压测时数据库到底发生了什么。每一次测试我们都在给数据库预热后，再执行5次。这个特殊参数配置我们测试了2次，结果值是稳定一致的。使用单线程的 sysbench 压测是最快的。我们将会进一步研究，以了解发生了什么。

First of all, we don’t really know what happened when sysbench was executed using 1 thread only. Each test was executed five times after a warmup run. This particular configuration was tested two times - results are stable: single-threaded workload was the fastest. We will be looking into it further to understand what happened.

除此之外，其余的结果都符合我们的预期。COMMIT_ORDER 是最慢的，特别是低并发时（2-8线程）。WRITESET_SESSION 通常比COMMIT_ORDER 更好，但是对于低并发流量，它比 WRITESET 慢。

Other than that, the rest of the results are in line with what we expected. COMMIT_ORDER is the slowest one, especially for low traffic, 2-8 threads. WRITESET_SESSION performs typically better than COMMIT_ORDER but it’s slower than WRITESET for low-concurrent traffic.

报告信息总结

第一个优点是显而易见的：如果主库工作负载较低，且从库复制速度倾向于变慢，那么只要你将主库升级到 8.0 就能从其改进的复制性能中获益。

The first advantage is obvious: if your workload is on the slow side yet your slaves have tendency to fall back in replication, they can benefit from improved replication performance as soon as the master will be upgraded to 8.0.

这里有两个注意事项：

1、这个特性是向后兼容的，所以 5.7 的从库也能从中获益；

2、请注意 MySQL 8.0 依然是 beta 版本，我们不鼓励您在生产环境中使用测试版，尽管你非常需要这些新功能。

Two notes here: first - this feature is backward compatible and 5.7 slaves can also benefit from it. Second - a reminder that 8.0 is still in beta state, we don’t encourage you to use beta software on production, although in dire need, this is an option to test.

这个特性不仅在你从库延迟时有作用，在你创建一个全新的从库或者重新配置一个已有的从库时，它也完全能跟得上。有了使用 "WRITESET" 模式的能力，配置一个新主机的过程将会变得更快。

This feature can help you not only when your slaves are lagging. They may be fully caught up but when you create a new slave or reprovision existing one, that slave will be lagging. Having the ability to use “WRITESET” mode will make the process of provisioning a new host much faster.

总而言之，这个特性带来的影响可能会超乎你想象。鉴于所有基准测试显示当 MySQL 处理低并发时性能较差，任何有助于加速在这种环境中复制的改进都将是巨大的进步。

All in all, this feature will have much bigger impact that you may think. Given all of the benchmarks showing regressions in performance when MySQL handles traffic of low concurrency, anything which can help to speed up the replication in such environments is a huge improvement.

如果你使用中继主库，这同样是你需要的特性。任何复制架构的中继主库都会在处理和执行事务时增加一些串行化信息。 —— 在真实的生产环境中，中继主库的工作负载几乎都是比主库的并行化程度低。

利用 writesets 来实现更好的并行化，不仅能提高中继主库的并行化程度，而且可以提高所有从库的并行化程度。甚至可以使用 8.0 中继主库（虽然要经过严格的测试以验证所有的功能正常使用）来提高从库（请注意 MySQL 5.7 从库虽然不能自己生成 writeset 数据，但是它能识别和使用writeset 数据）的复制性能。当然，从 8.0 复制到 5.7 听起来有点诡异（不仅仅是因为 8.0 还是 beta 版本）。在某些情况下，这可能会起作用，并可以加快 5.7 从库上的 CPU 利用率。

If you use intermediate masters, this is also a feature to look for.Any intermediate master adds some serialization into how transactions are handled and executed . - in real world, the workload on an intermediate master will almost always be less parallel than on the master.Utilizing writesets to allow better parallelization not only improves parallelization on the intermediate master but it also can improve parallelization on all of its slaves.It is even possible (although it would require serious testing to verify all pieces will fit correctly) to use an 8.0 intermediate master to improve replication performance of your slaves (please keep in mind that MySQL 5.7 slave can understand writeset data and use it even though it cannot generate it on its own).Of course, replicating from 8.0 to 5.7 sounds quite tricky (and it’s not only because 8.0 is still beta). Under some circumstances, this may work and can speed up CPU utilization on your 5.7 slaves.

MySQL 复制的其他变化

除了最有趣的 writesets 新特性，MySQL 8.0 中关于 MySQL 复制的其他变化也是值得关注的。我们来看看其他的一些重要变化。

如果你碰巧使用了一个比 5.0 版本还老的MySQL，请注意 MySQL 8.0 将不再支持它的二进制日志格式。我们非常不建议采用这种 MySQL 复制方式，但是如果你真的在复制架构中使用一些非常老的 MySQL 版本，那真的是时候去升级了。

Introducing writesets, while it is the most interesting, it is not the only change that happened to MySQL replication in MySQL 8.0. Let’s go through some other, also important changes. If you happen to use a master older than MySQL 5.0, 8.0 won’t support its binary log format. We don’t expect to see many such setups, but if you use some very old MySQL with replication, it’s definitely a time to upgrade.

为了尽可能地保证复制架构中的MySQL数据库崩溃恢复时数据库的安全性，MySQL 8.0 中一些默认值已更改：

• master_info_repository 和 relay_log_info_repository 默认设置为 TABLE；

• expire_log_days 的默认值也变成了 30；

• 除了 expire_log_days 之外，还添加了一个新的参数 binlog_expire_log_seconds，它允许更细粒度的 binlog 轮换策略；

• 二进制日志中添加了一些额外的时间戳，使复制延迟时可以更好地被观察，同时还引入了微秒级别的粒度。

Default values have changed to make sure that replication is as crash-safe as possible: master_info_repository andrelay_log_info_repository are set to TABLE. Expire_log_days has also been changed - now the default value is 30. In addition to expire_log_days, a new variable has been added, binlog_expire_log_seconds, which allows for more fine-grained binlog rotation policy. Some additional timestamps have been added to the binary log to improve observability of replication lag, introducing microsecond granularity.

然而，这并不是所有 MySQL 8.0 复制相关的心功能的完整列表。如果你想了解更多信息，可以查看 MySQL changelogs（https://dev.mysql.com/doc/relnotes/mysql/8.0/en），以确保您已经查看到所有相关信息 —— 到目前为止，所有 8.0 版本都添加了这些特性。

By all means, this is not a full list of changes and features related to MySQL replication. If you’d like to learn more, you can check the MySQL changelogs. Make sure you reviewed all of them - so far, features have been added in all 8.0 versions.

正如你所看到的，MySQL 复制仍然在变化而且越来越好。正如我们刚才所说的那样，MySQL 复制变化是一个缓慢的进程，但是 MySQL 复制的前景是非常好的。我们很高兴看到组复制的工作成果能在常规的 MySQL 复制中使用并且用得很好。

As you can see, MySQL replication is still changing and becoming better. As we said at the beginning, it has to be a slow-paced process but it’s really great to see what is ahead. It’s also nice to see the work for Group Replication trickling down and reused in the “regular” MySQL replication.