jcole’s weblog

Just a little heads-up and a bit of MySQL-related technical content for all of you still out there following along…

At Proven Scaling, we take on MySQL performance problems pretty regularly, I’m often in need of good tools to characterize current performance and find any issues. In the database world, you’re really looking for a few things of interest related to I/O: throughput in bytes, requests, and latency. The typical tool to get this information on Linux is iostat. You would normally run it like iostat -dx 1 sda and its output would be something like this, repeating every 1 second:


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 8.00 0.00 4.00 0.00 96.00 0.00 48.00 24.00 0.06 15.75 15.75 6.30


Most of the output of iostat is interesting and reasonable for its intended purpose, which is as a general purpose way to monitor I/O. The really interesting things for most database servers (especially those in trouble) are:

• avgrq-sz — Average request size, in kilobytes.
• avgqu-sz — Average I/O queue length, in requests.
• await — Average waiting time (in queue and scheduler) before servicing a request, in milliseconds.
• svctm — Average total service time for I/O requests, in milliseconds. This includes await, so should always be higher than await. This is the most interesting number for any write-heavy transactional database server, as it translates directly to transaction commit time.
• %util — Approximate percent utilization for the device.

There are one major problem with using iostat to monitor MySQL/InnoDB servers: svctm and await combine reads and writes. With a reasonably configured InnoDB, on a server with RAID with a battery-backed write cache (BBWC), reads and writes will have very different behaviour. In general, with a non-filled cache, writes should complete (to the BBWC) in just about zero milliseconds. Reads should take approximately the theoretical average time possible on the underlying disk subsystem.

I’ve often times found myself scratching my head looking at a non-sensical svctm due to reads and writes being combined together. One day I was perplexed enough to do something about it: I opened up the code for iostat to see how it worked. It turns out that the core of what it does is quite simple (so much so, I wonder why it’s C instead of Perl) — it opens /proc/diskstats, and /proc/stat and does some magic to the contents.

What I really wanted is a histogram of the reads and writes (separately, please!) for the given device. I hacked up a quick script to do that, and noticed how incredibly useful it is. I recently had to extend it to address other customer needs, so I worked on it a bit more and now it looks pretty good. Here’s an example from a test machine (so not that realistic for a MySQL server):

util:   1.27% r_ios:     0  w_ios:     1  aveq:     0,
ms : r_svctm                     : w_svctm
 0 :                             :
 1 :                             :
 2 :                             :
 3 : x                           :
 4 : x                           :
 5 : xxx                         :
 6 : xxxx                        :
 7 :                             :
 8 : x                           : x
 9 : x                           : xx
10 : x                           : xxxxx
11 :                             : xxxxxxxxxxxxxxx
12 :                             : xxxxxxxxxxxxxxxxxxxxxxxxx
13 : xx                          : xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
14 :                             : xxxxxxxxxxxxxxxxxxxxx
15 : xx                          : xxxxxxxxxxx
16 : x                           : xxxxx
17 : x                           : xxxxxx
18 :                             : xxxx
19 : x                           : xx
20 :                             : x
21 :                             : x
22 :                             : x
23 :                             : x
24 :                             : x
25 :                             :
26 :                             :
27 :                             :
28 : x                           :
29 :                             :
30 :                             :
++ : 0                           : 250

It uses Curses now to avoid redrawing the entire screen, and I’ve got a ton of ideas on how to improve it. I have a few more must-haves before I release it formally to the world, but I wonder what more features people would want from it. It is Linux-only for the foreseeable future.

What do you think?

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This entry was posted on Tuesday, May 8th, 2007 at 22:17:19 and is filed under MySQL, MySQL Tips, Proven Scaling, Technology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.