[rrd-users] HELP I dont understand RRA's

[tony at specialistdevelopment.com]

Thankyou :)

A great help.

Tony

Quoting Simon Hobson <linux at thehobsons.co.uk>:

> tony at specialistdevelopment.com wrote:
>
>> Im really sorry to ask this as im probably being very stupid...Im  
>> new to RRD tool
>
> Yes, it can be a bit intimidating to start with.
>
>> I have followed tutorials, examples etc, but to finally get my head  
>> around it im trying to do it on my own, but i come unstuck, when it  
>> comes to RRA's in the RRD creation.
>>
>> Can anyone explain in really, really simple terms what they are?
>
> Have you read this one ?
> http://www.vandenbogaerdt.nl/rrdtool/tutorial/rrdcreate.php
>
> Anyway, in short, there are two things you need to define in an RRD  
> database :
>
> 1) A DS for each source of data
> 2) An RRA function for each combination of resolution, timescale,  
> and aggregation function required.
>
> You cannot graph a DS directly, only an RRA aggregate of it. Each  
> RRA specifies :
> How many primary data points (PDP) to aggregate into a consolidated  
> data point (CDP).
> How many consolidated data points to keep.
> What aggregation function to use.
>
> So lets take an example. I collect data with a 5 minute (300s) step  
> time. So that means my PDPs are at 00:05, 00:10, 00:15, and so on.  
> This isn't really that useful if I only want a monthly overview. I  
> might for example, consolidate 24 PDPs into a CDP - that gives me 2  
> hours/CDP. If I now plot a graph that is 273 pixels wide, I will get  
> one pixel per CDP and the graphing process needs few resources to  
> run as the data is already at the right resolution.
>
> However, for a more detailed graph of the data over the last day, I  
> would probably want to keep a consolidation with only 1 PDP per CDP.  
> Thus I'd get 288 CDPs per day and could plot a graph 288 pixels wide  
> to cover one day in detail.
>
> Neither of the above could **NOT** be done by simply keeping every 5  
> minute sample - but lets consider longer periods. If I want to be  
> able to graph data over a year, then I'd need to keep over 100k 5  
> minute samples. That requires a lot of storage, and a lot of  
> processing every time you want to draw a graph. Since in most cases  
> we aren't interested in the fine detail from a year ago, we can  
> define multiple RRAs to suit. For example, I keep 5 minute samples  
> for 2 days, 30 minute consolidated data for 2 weeks, 2 hours  
> consolidated data for 2 months, and  24hour consolidated data for 2  
> years.
>
> Once you've defined the level of detail and length of time for each  
> RRA, then RRD tool takes care of updating each one as required. Thus  
> storage and processing requirements for graphing are kept to  
> reasonable levels.
>
>
> The last element is what type of aggregation function to use. You  
> may want to keep multiple RRAs - a typical case might be to keep  
> both an average and maximum. For things like data traffic, for  
> sizing links etc you need to know maximums, but for assessing how  
> much overall traffic you are using, you need to keep averages.
>
> Imagine a situation where a service uses 24mbps for one hour during  
> a day. You need to know that 24mbps has been used so that you know  
> the link needs to cope with 24mbps - so you need a maximum RRA to  
> capture that.
> On the other hand, if you are looking at volumes of traffic (perhaps  
> you are billing a customer for data transferred to/from a server),  
> then you'd want to keep the average (1mbps over a 24 hour period).  
> Again, your options are to store consolidated data (ie average over  
> each day - 1mbps * 24hr), or you can store all the detailed data and  
> work it out later (ie 24mbps * 1hr plus 0mbps * 23hr).
>
>
> -- 
> Simon Hobson
>
> WANTED: "Software CD ROM Kit" for Canon CLBP 360-PS printer (Canon  
> part no RH6-3612, or possibly RH6-3810, or RH6-3610 might do). I've  
> a dead HD and need this CD so I can replace the disk and re-install  
> the printer OS on it. If anyone knows where I might get hold of one  
> I'd be grateful - requests to Canon drew a blank, it's been out of  
> support for years.
> Alternatively, if anyone has one of these and would let me image  
> their hard disk ...
>
> Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
> author Gladys Hobson. Novels - poetry - short stories - ideal as
> Christmas stocking fillers. Some available as e-books.
>