Percona Toolkit’s  pt-table-checksum performs an online replication consistency check by executing checksum queries on the master, which produces different results on replicas that are inconsistent with the master – and the tool pt-table-sync synchronizes data efficiently between MySQL tables.

The tools by default use the CRC32. Other good choices include MD5 and SHA1. If you have installed the FNV_64 user-defined function, pt-table-sync will detect it and prefer to use it, because it is much faster than the built-ins. You can also use …

I explored two interesting topics today while learning more about Postgres.

Partial page writes

PostgreSQL’s partial page write protection is configured by the following setting, which defaults to “on”:

full_page_writes (boolean)

When this parameter is on, the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint… Storing the full page image guarantees that the page can be correctly restored, but at a price in increasing the amount of data that must be written to WAL. (Because WAL replay always starts from a checkpoint, it is sufficient to do this during the first change of each page after a checkpoint. Therefore, one way to reduce the cost of full-page writes is to increase the checkpoint interval parameters.)

Trying to reduce the cost of full-page writes by increasing the checkpoint interval highlights a compromise. …

Over in Maatkit-land, Daniel Nichter and I practice test-first programming, AKA test-driven development. That is, we write tests for each new feature or to catch regressions on each bug we fix. And — this is crucial — we write the tests before we write the code.* The tests should initially fail, which is a validation that the new code actually works and the tests actually verify this. If we don’t first write a failing testcase, then our code lacks a very important guarantee: “if you break this code, then the test case will tell you so.” (A test that doesn’t fail when the code fails isn’t worth writing.)

Most of the time when I do this, I write a test, it fails because I haven’t written any code yet, and I then go do some kind of …