Here on the Percona Support team we often ask customers to retrieve disk stats to monitor disk IO and to measure block devices iops and latency. There are a number of tools available to monitor IO on Linux. iostat is one of the popular tools and Percona Toolkit, which is free, contains the pt-diskstats tool for this purpose. The pt-diskstats tool is similar to iostat but it’s more interactive and contains extended information. pt-diskstats reports current disk activity and shows the statistics for the last second (which by …[Read more]
Earlier this month I wrote about vmstat iowait cpu numbers and some of the comments I got were advertising the use of util% as reported by the iostat tool instead. I find this number even more useless for MySQL performance tuning and capacity planning.
Now let me start by saying this is a really tricky and deceptive number. Many DBAs who report instances of their systems having a very busy IO subsystem said the util% in vmstat was above 99% and therefore they believe this number is a good indicator of an overloaded IO subsystem.
Indeed – when your IO subsystem is busy, up to its full capacity, the utilization should be very close to 100%. However, it is perfectly possible for the IO subsystem and MySQL with it to have plenty more capacity than when utilization is showing 100% – as I will show in an …[Read more]
Next Wednesday, I will present a webinar about MySQL performance profiling tools that every MySQL DBA should know.
Application performance is a key aspect of ensuring a good experience for your end users. But finding and fixing performance bottlenecks is difficult in the complex systems that define today’s web applications. Having a method and knowing how to use the tools available can significantly reduce the amount of time between problems manifesting and fixes being deployed.
In the webinar, titled “Optimizing MySQL Performance: Choosing the Right Tool for the Job,” we’ll start with the basic top, iostat, and vmstat then move onto advanced tools like GDB, Oprofile, and Strace.
I’m looking forward to this webinar and invite you to join us April 16th at 10 a.m. Pacific time. You can learn more and also …[Read more]
These days I spend more time looking at /proc/diskstats than I do at iostat. The problem with iostat is that it lumps reads and writes together, and I want to see them separately. That’s really important on a database server (e.g. MySQL performance analysis).
It’s not easy to read /proc/diskstats by looking at them, though. So I usually do the following to get a nice readable table:
- Grep out the device I want to examine.
- Push that through “rel” from the Aspersa project.
- Add column headers, then format it with “align” from the same project.
Here’s a recipe. You might want to refer to the kernel iostat documentation too.
chmod +x rel align
while sleep 1; do grep sdb1 …
I like to write tools that make hard things easy, when possible. By and large, MySQL is easy and simple. But some simple things are too hard with MySQL. I want to change that, at least for the things that matter the most to me, and which I think I know how to fix.
I will probably write a lot about this. I have already written a number of rants blog posts about the lack of instrumentation in MySQL, and that is where I’ll probably continue to put most of my energy.
To begin with, imagine this simple scenario. You are a remote DBA. Your client says “New Relic is showing periods of slow response time from the database.” You connect to MySQL at the command line and try to troubleshoot. How do you catch the problem in action, from within the database itself? The following are no good:
- It doesn’t count to see the problem two minutes later by observing the application tier, as New Relic does. That’s too late, and …
System administrators familiar with the Linux operating system
use the tools in the 'procps' toolset all the time. Tools which
read from /proc include top, iostat, vmstat, sar and others. The
files in /proc contain useful information about the performance
of the system. Most of the files are documented in the Linux kernel documentation. You can also check
man 5 proc.
Most performance monitoring tools invoke other tools like iostat to collect performance information instead of reading from the /proc filesytem itself. This begs the question, what can you do if you don't have access to those tools? Perhaps you are using a hosted Linux database and have no access to the underlying shell to execute tools like iostat or top? How could you gather information about the performance of the actual system without being allowed to run the tools?
I’ve released version 1.1.6 of the Better Cacti Templates project. This release includes a bunch of bug fixes (but not all of them!) and two new sets of graphs. One set is for disk I/O on GNU/Linux, and the other is a new set of templates for OpenVZ. I’m looking for feedback on both of those. This release also has a bunch of code-level features: much better test coverage (hooray!), and a refactored ss_get_by_ssh.php that makes it much easier to create new graphs and templates. The SSH-based templates also take advantage of the same caching as the MySQL templates, which makes them a lot more efficient.
There are upgrade instructions on the project wiki for this and all releases. There is also a comprehensive tutorial on …[Read more]
iostat is one of the most important tools for
measuring disk performance, which of course is very relevant for
database administrators, whether your chosen database is
Postgres, MySQL, Oracle, or anything else that runs on GNU/Linux.
Have you ever wondered where statistics like await (average wait
for the request to complete) come from? If you look at the disk
statistics the Linux kernel makes available through files such as
/proc/diskstats, you won’t see await there. How does iostat
compute await? For that matter, how does it compute the average
queue size, service time, and utilization? This blog post will
show you how that’s computed.
First, let’s look at the fields in /proc/diskstats. The order and location varies between kernels, but the following applies to 2.6 kernels. For reads and writes, the file contains the number of …[Read more]
Here is a situation I’ve run into a few times when dealing with mysql databases. We’re trying to run a one-off query against a high-traffic, large table and the WHERE condition is against a non-indexed field. Let’s say our table is 5GB in size. We issue the following:
SELECT count(*) from five_gb_myisam_table WHERE
non_idx_field = 'asdf';
and we wait…
and wait some more.
5GB is not a small table, but this ideally should not take more than a few minutes on a relatively modern system.
iostat is your friend
In cases like this,
iostat -x 5 is your friend.
While the query was running, this was a typical 5 second
avg-cpu: %user %nice %sys %iowait %idle
2.30 0.00 1.30 96.40 0.00
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz
avgqu-sz await svctm %util
sda 0.00 0.70 …
Quick... You have to let everyone (boss, biz dev, customer service, and random bean counters) know why everything is moving slowly! Of course, rarely do people define what "everything" is, and what type of slowness is occuring. But, in the face of customer service agents that cannot work because their pages will not render, generally all eyes are on the famed-dba-of-the-minute.So, with 7 people