The mk-slave-prefetch tool is designed to try to execute a read-only approximation of the write workload that the replica is about to have to perform. It is meant to do this just a little bit before the replication thread (which can only be true if replication is lagging), so that when the replica replays writes to execute replication, it doesn’t have to wait for disk I/O.

Keeping caches warmed up for production traffic requires that the read workload, which does not flow through relay logs, is executed on the server. If you point mk-slave-prefetch at a server, you’re just double-executing the write workload in a read-only fashion.

Perhaps an example will clarify. Suppose that a master database has a bunch of writes to WritableTable, and lots and lots of reads to ReadableTable. On the replica, the only queries in the relay log will be on WritableTable. And what will mk-slave-prefetch be doing? Executing SELECT queries against WritableTable. No traffic on ReadableTable at all. If you switch to use the replica as the primary server, ReadableTable’s data won’t be in the caches.

To actually warm up the replica, you need to replay the queries against the ReadableTable. You can only find those by looking at the master. You can read its query logs, or watch its processlist, or capture TCP traffic, or any other method of capturing read traffic. There is a feature built into mk-query-digest to help you capture and replay these against the replica: –execute.

A parting note: mk-slave-prefetch is an extremely niche tool that generally doesn’t help replicas keep up with their masters. Only in very special cases is the hardware, data, and workload suitable for what it does.

Further Reading:

• Pop quiz: how can one slave break another slave
• Why MySQL says the server is not configured as a slave
• How to know if a MySQL slave is identical to its master
• How to measure MySQL slave lag accurately
• MySQL disaster recovery by promoting a slave

Further Reading:

• Want to hack Maatkit and Aspersa? We’re hiring
• Aspersa, a new opensource toolkit
• Four companies to sponsor Maatkit development
• How Maatkit benefits from test-driven development
• Aspersa gets a user manual

What’s this tool good for? Well, imagine that you’re a big MySQL user and you hire a new developer. Now you need to bring the new person up to speed with your environment. Or, you want to understand where the data in some table actually comes from. Or, you want to drop a column, but you’re not sure where that data is used and what other code will be affected. Or you want to find all SQL statements that modify a table. Wouldn’t it be nice to have a graph of all your tables and the data flows between them? With this tool you can parse the flow of data in SQL statements, in terms of Table-From → Table-To, and print the results, annotated by the statement’s fingerprint.

The client who sponsored the development of this tool is using it as an auditing mechanism, for some of the purposes I just mentioned, and also to help enforce their SQL coding standards. It can be used for a lot more than that, though. I haven’t done this yet, but it should be easy to write some quick 5-line script to transform it into graphviz format and produce graphs from it, or import into a table that represents edges and run queries against it, and so on. (The client is doing some of those things, but they aren’t asking me to help, so I’m taking their word for it that the output format they chose is easily amenable to these tasks.)

Further Reading:

• New Maatkit tool to compute index usage
• Analyze and optimize memcached usage with Maatkit
• Try mk-query-advisor, a new Maatkit tool
• Aspersa’s mysql-summary tool
• MySQL Toolkit’s Show Grants tool 0.9.1 released

For Maatkit, we solved this problem by making the tool read its own source code and generate command-line options, default values, behaviors, dependencies, data types, and so on directly from its own embedded documentation. This is the same documentation that gets converted into man pages. So when you run the program, view its documentation, ask it for –help, or whatever you do, you get the same information. The documentation is part of the program, and if you change the documentation, you change the program.

For a while I was very unhappy with using Perl to reach outside the boundaries of Perl. It turns out that executing another program, capturing its output, controlling it, capturing its return code, etc is very buggy. So I started to write scripts that need this capability in bash, because it is obviously very good at these tasks. But it’s a bit harder to handle command-line options in bash, and the tools available for it differ or are unavailable on various platforms. So I ended up with usage information in a block of text, and program options defined in program code. Yuck!

I fixed that recently. I wrote a short script that reads the usage text and generates code to implement the options, including default values and options that are constrained to certain valid inputs. Life is good again.

Further Reading:

• How to create a VB6 console program
• How to leave a program running after you log out
• A shell for a new Perl MySQL program
• How to add paragraph spacing in Google Docs
• New support options for innotop

Further Reading:

• New Ruby Conference: Ruby on Ales 2011
• The little-known Maatkit man page
• Get Maatkit fast from the command line
• Is Maatkit notable enough for Wikipedia now?
• New Maatkit tool: mk-table-usage