tuning

Analyzing a MySQL slow query log with pt-query-digest

There are times when you may notice your MySQL or MariaDB database server getting very slow. Usually, it's a very stressful time, as it means your site or application is also getting very slow since the underlying database is slow. And then when you dig in, you notice that logs are filling up—and in MySQL's case, the slow query log is often a canary in a coal mine which can indicate potential performance issues (or highlight active performance issues).

But—assuming you have the slow query log enabled—have you ever grabbed a copy of the log and dug into it? It can be extremely daunting. It's literally a list of query metrics (time, how long the query took, how long it locked the table), then the raw slow query itself. How do you know which query takes the longest time? And is there one sort-of slow query that is actually the worst, just because it's being run hundreds of times per minute?

Slow Ansible playbook? Check ansible.cfg!

Today while I was running a particularly large Ansible playbook about the 15th time in a row, I was wondering why this playbook seemed to run quite a bit slower than most other playbooks, even though I was managing a server that was in the same datacenter as most of my other infrastructure.

I have had pipelining = True in my system /etc/ansible/ansible.cfg for ages, and initially wondered why the individual tasks were so delayed—even when doing something like running three lineinfile tasks on one config file. The only major difference in this slow playbook's configuration was that I had a local ansible.cfg file in the playbook, to override my global roles_path (I wanted the specific role versions for this playbook to be managed and stored local to the playbook).

So, my curiosity led me to a more thorough reading of Ansible's configuration documentation, specifically a section talking about Ansible configuration file precedence:

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