Database tuning: ratio vs. rate

Baron makes an excellent point in Why you should ignore MySQL’s key cache hit ratio — ratio is not the same as rate. Furthermore, rate is [often] the important thing to look at.

This is something that, at Pythian, we internalized a long time ago when thinking about MySQL tuning. In fact, mysqltuner 2.0 takes this into account, and the default configuration includes looking at both ratios and rates.

If I told you that your database had a ratio of temporary tables written to disk of 20%, you might think “aha, my database is slow because of a lot of file I/O caused by writing temporary tables to disk!”. However, that 20% ratio may actually mean a rate of 2 per hour — which is most likely not causing excessive I/O.

To get a sense of this concept, and also how mysqltuner works, I will show the lines from the mysqltuner default configuration that deal with temporary tables written to disk. The format is that the fields are separated by three pipes (|||), and the fields are:

label
threshold check
formula
recommendation if “threshold check” is met

Here is the line from the default configuration file that calculates the rate of temporary tables written to disk:

% temp disk tables|||>25|||Created_tmp_disk_tables / (Created_tmp_tables + Created_tmp_disk_tables) * 100|||Too many temporary tables are being written to disk.  Increase max_heap_table_size and tmp_table_size.

mysqltuner will parse that as:

if
the value of Created_tmp_disk_tables/(Created_tmp_tables + Created_tmp_disk_tables)*100
>25
then print out the last field.

So that means that a ratio of 25% or more is the threshold. But we found that many clients have a ratio much less than 25%, but still had excessive temporary tables written to disk. So the default configuration also contains a rate calculation of temporary tables written to disk:

temp disk rate|||=~ /second|minute/|||&hr_bytime(Created_tmp_disk_tables/Uptime)|||Too many temporary tables are being written to disk.  Increase max_heap_table_size and tmp_table_size.

mysqltuner will parse that as:

if
the value of &hr_bytime(Created_tmp_disk_tables/Uptime)
matches “second” or “minute”
then print out the last field.

The hr_bytime() function in mysqltuner takes a number that is a per-second rate and makes it “human readable” (hence “hr”) by returning the order of magnitude at which the value is >1. For example:

hr_bytime(2) returns “2.0 per second”
hr_bytime(0.2) returns “12.0 per minute”
hr_bytime(0.02) returns “1.2 per minute”
hr_bytime(0.002) returns “7.2 per hour”
hr_bytime(0.0002) returns “17.28 per day”

Certainly, 0.02 looks small, but “12 per minute” is a better metric for a DBA to understand the problem.

Because the configuration file for mysqltuner 2.0 contains the threshold and check, it is fairly simple to change what the threshold is, and to check both rates and ratios. mysqltuner also allows you to output in different formats (currently there’s “pretty” and “csv”, but it’s easy to add a perl subroutine to do something different with the output), which makes it ideal for doing regular tuning checks for what is most important for you.

Pythian uses it on one client to provide weekly reports, which we add to a spreadsheet so that differences are easy to see. (yes, output directly to a database is on the “features we want to add” — mysqltuner is just a perl script, so if anyone in the community wants to add it, they can create a branch and request the feature to be added into the main trunk…it is all on launchpad, at https://launchpad.net/mysqltuner, so community contributions are recommended and encouraged.)

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