[rrd-users] Design an RRD for the following data

[Goldburt, Dan]

Friends,

Apologies if this has been discussed elsewhere, it seems to me like a
common enough problem but I couldn't find a solution described anywhere.
I'm trying to graph stock market volume. As you can see, the data is a
counter that gets updated roughly every 30 minutes and resets at the end
of the day:

TIME    	(UNIX TIME)			NASDAQCOMP VOL 
Tue Apr 24 15:00:05 2007 (1177441205)   1798518651
Tue Apr 24 15:30:05 2007 (1177443005)   1952143339
Tue Apr 24 16:00:04 2007 (1177444804)   2214507628
Tue Apr 24 16:30:03 2007 (1177446603)   2306049386
Tue Apr 24 17:00:02 2007 (1177448402)   2332135566
Tue Apr 24 17:30:02 2007 (1177450202)   2342303973
Tue Apr 24 18:00:03 2007 (1177452003)   2345873356
Tue Apr 24 18:35:01 2007 (1177454101)   2347341668
Wed Apr 25 10:00:09 2007 (1177509609)   401969956
Wed Apr 25 10:30:07 2007 (1177511407)   666108171
Wed Apr 25 11:00:10 2007 (1177513210)   858504492

I'd like to define an RRA with one DS and be able to generate two kinds
of graphs - trending market activity during the day (trades/sec), and
trending daily volumes (total trades each day) to see if the market is
getting busier in the long run. I've thought of several different
approaches and none of them work sufficiently.

1. I define my DS as a COUNTER. This would give me trades per second,
but how would I handle the counter reset after every day? Also, I see no
way to get the total volume for a day out of the RRA. If my DS type
performed some kind of rate of change calculation, how do I integrate
the DS to get the actual values back? 

2. I define my DS a a GAGUE. Is there a way to graph deltas? How can I
define a CDEF that calculates the difference between two consecutive
polls (I don't even necessarily need to divide by the step size - a
graph of trades per 1/2 hour is just as good as trades per second). For
the second graph, I would want to keep the last value for each day,
since that represents the total. Can I define an RRA to do that? If I
take the LAST of the past 48 PDP (48 half hours in a day), that doesn't
mean I will get the last value unless I initially start polling in the
beginning of the day.

3. I define my DS as a GAGUE, but do some pre-processing before
updating. I could subtract the last value from the current value and
just update the deltas. This helps with the first graph, but now I need
an RRA with a CF of SUM, which doesn't exist.

I really don't want to define more than one DS and deal with the
complexity of updating multiple RRD's with the same data. Please, any
advice would be appreciated.

Thanks,
Dan Goldburt