[hobbit] Client interval question

Scott Walters scott at PacketPushers.com
Thu Dec 15 09:41:49 CET 2005

> Well, we could take this the other way, and say that we only wanted to
> run our tests once every 10 minutes, because it was causing too much
> overhead to run the tests every 5 minutes. How would we deal with  
> that?

Same issue, the server needs to know the sampling rate when the RRD  
is created.

> I think there are benefits, on the client side, the client should pass
> the frequency which it is calling the tests at, to them, so that for
> example, the vmstat test can adjust how long it will run for to either
> 600 seconds, or 60 seconds, or whatever else is needed.

As long as those never need to change, that wouldn't be too bad.  But  
then you run into the display logic needing help depending on the  
granularity of the data/RRAs in the RRDs.

> Further to that, there is some additional work (which I feel is the  
> real
> place that all the work is involved, the client side stuff would be
> quite simple, or so it sounds).

> So, the question becomes, how difficult is it to convert an rrd file,
> which was initially created to store data-points every 300 seconds,  
> such
> that we can now store data-points every X seconds?

I am not aware of a way to  change the granularity of RRAs (the  
things inside the RRDs) once they are created.  You'd have to rrdtool  
export; create a new rrd with different RRA's, then rrdtool import.    
Basically export/import the database.  You can't even add an RRA to  
an existing RRD.

> The second part to this question, is how does hobbit know how  
> frequently
> you want to send your reports? ie, it can't be based on 'however often
> they are received', because that value would change very  
> frequantly, ie,
> the reports are done every 300 seconds, but by the time the report is
> submitted/processed by hobbit, it might be 1 or 2 seconds late/early
> compared to last time..... Could hobbit server 'learn' the frequency
> from the client (which is where this is configured anyway), because  
> the
> client would report that value to the server as a part of the vmstat
> output?

Yes, but that is not what makes all this really hard, it's the server  
logic.  I can think of  ways to do it, but it would involve a lot of  
changes to the server side parsing, many small client changes,  
restructuring/redefining existing rrds, and some potentially hairy  
presentation logic to make the server smarter about what to show  
based on what is in the RRD.   I wrote larrd with Christian, and I  
can tell you, this would not be a weekend hack.

Time Series Data (telemetry data) is all about data on regular  
intervals.  Changing that regular interval is a very significant thing.

> or anything else, I have no interest in any of it, I'm just stating a
> simple fact of life (you want something done, you can't do it, so find
> someone who can do it, and motivate them to the point where they  
> will do
> it whether they want to or not)...

The real trick there is convincing them they want to do it.  Forcing  
someone to do something might work, but is no good over the long term.
> Agreed, it would be nice to not have to run hobbit plus something else
> when they are both collecting the same data (just a different
> frequency).

I'm tellin' ya: vmstat 5


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