pt-online-schema-change is simple to use, but internally it is complex. Baron’s webinar about pt-online-schema-change hinted at several of the tool’s complexities. Consequently, users often want to know before making changes what pt-online-schema-change will do when it runs. The tool has two options to help answer this question: –dry-run and –print.
MySQL’s SHOW STATUS and SHOW VARIABLES commands (or queries against the corresponding INFORMATION_SCHEMA tables) don’t always show what they say. In particular, SHOW STATUS contains several rows that aren’t status-related, but are really configuration variables in my opinion (and it is an opinion — sometimes the difference isn’t black and white).
Here’s a short list of some status counters that I think are really better off as configuration variables:
Last night I wrote about trending data with a moving average, and then after I went to bed, I realized I made a mistake on the chart I showed. I calculated α for the exponentially weighted moving average so that the average age of metrics approaches 60 samples as time approaches infinity, and I plotted that on the same chart with a 60-sample simple moving average.
In my recent talk at Surge and Percona Live about adaptive fault detection (slides), I claimed that hardcoded thresholds for alerting about error conditions are usually best to avoid in favor of dynamic or adaptive thresholds. (I actually went much further than that and said that it’s possible to detect faults with great confidence in many systems like MySQL, without setting any thresholds at all.)
MySQL command line utilities have an interesting property – if you only use the prefix of the option or command it will go over the list of available command and if there is only one command with matching prefix it will execute it with no warnings or any kind, otherwise it will report the error. For example mysqladmin e works as there is only one command “extended-status” which starts with “e” mysqladmin f however does not work because there are multiple commands which start with “f”.
Within hours of my post about meeting the MySQL community in Barcelona, we got several offers to help, and within one day, an event was created and agreed upon.