[Xymon] diskstat.sh/RRD oddity

[W.J.M. Nelis]

Hello,

> This is just a comment on an oddity with respect to diskstat.sh and RRD.
>
> We make pretty heavy use of the diskstat.sh script, which I believe I
> downloaded from xymonton. When I installed it I used the standard
> clientlaunch.cfg stanza for the configuration and everything worked great.
>
> I was called to task today because we have been having some disk io issues
> on the RHEL VMs and someone was looking at the trend graphs for some
> servers to see if there was anything they could learn and they noticed that
> beginning at about 4pm local time on Monday the graphs for the number of
> sectors written per second on a couple of file systems on several VMs
> jumped from the 10 to 20 range to the 300 to 340 range and stayed there.
> The graph for number of disk writes per second had a corresponding jump up
> to about 40 or 50 from close to zero.
>
> In analyzing the data I discovered that the file system that was displaying
> this behavior is the same file system to which the diskstat.sh script is
> writing its temp files. It appears that for some reason, starting at 4pm on
> Monday the 5 minute test interval and the 5 minute average for RRD got in
> sync and all it was seeing was the data point that corresponded to its own
> writing activity and RRD was using it for the entire 5 minute average (of
> course, that's what RRD does).
The diskstat.sh script collects a sample at each invocation. Thus the 
measurements cover only a fraction of time. As you seem to be using RHEL, 
you could use /proc/diskstats to get the same data, but in stead of a 
statistical sample, you will get averages since the last invocation 
(measurement). That would prohibit the "positive" interference described 
above, in which you measure your own measurement activities.

Regards,
   Wim Nelis.




******************************************************************************************************************

The NLR disclaimer is valid for NLR e-mail messages.

This message is only meant for providing information. Nothing in this e-mail message amounts to a contractual
or legal commitment on the part of the sender.
This message may contain information that is not intended for you. If you are not the addressee or if this
message was sent to you by mistake, you are requested to inform the sender and delete the message.
Sender accepts no liability for damage of any kind resulting from the risks inherent in the electronic
transmission of messages.
 
******************************************************************************************************************