[rrd-users] Re: Inconsistencies between graph and fetch

[Tobias Oetiker]

Yesterday Philip Molter wrote:

 |
 | On Wed, Feb 28, 2001 at 01:04:37AM +0100, Alex van den Bogaerdt wrote:
 | : Fetch does show you the startout value.  This value however is
 | : describing the period of time that is not inside your query.
 | : This may be considered a bug or a feature, depending on how you
 | : look at it.
 |
 | What I'm saying is its a bug.  If these discrete moments in time
 | are supposed to be taken not as discrete moments but rather as
 | representative of entire intervals (which I don't particularly have
 | a problem with), then the data retrieval methods should be consistent
 | in that representation.  It makes no sense to request the intervals
 | for a ten second time period and receive three "intervals" and then
 | request the average for that time period and find that it only
 | averages two of those "intervals".

the nature of the rrdtool data storage is such, that it always
stores a value together with a timestamp. the value is valid for
the interval between the timestamp and the previous timestamp ...
when you ask for an average between two timestamps the answer must
be the average ofthe data INSIDE the area defined by the timestamps

rrdtool fetch alsways returns you enough data to cover the whole
interval you requested data for ... you will get one data element more
than you requeste, at the beginning of the interval but this is
primarily because you need its timestamp to make sense of the data
for the second timestamp ... (given the timestamps are equidistant,
so you could argue that you do not need it. and if you do, so just
drop the first line of the fetch response and be happy)

tobi

 | I mean, RRD's confusing enough.
 |
 | * Philip Molter
 | * DataFoundry.net
 | * http://www.datafoundry.net/
 | * philip at datafoundry.net
 |
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