[rrd-users] Re: How to use old interface rrd data on new interface without peak ?

[Alex van den Bogaerdt]

Ytsejam wrote:

> Now, I got a new router, so I change rrd to collect from the new router & a new interface.
> I also copy the old serial interface .rrd & .meta file and rename to the new interface, so that I can keep the history data.
> It works, but because the counter is different between old router/interface and new one.
> When rrd collect data from the new interface and update into the old .rrd file,
> it generates a huge peak!

This is a variant on the counter-reset problem.  The most easy way to
circumvent this is to insert some unknowns in the input.  This should
generate two unknown intervals.

The rates are calculated by subtracting the previous value from the
current one.  Indeed, if you change devices (or reset the counters
for that matter) you can have a problem.  However, if you subtract
the unknown value from the current value (which is what happens if
you insert an unknown followed by the new value) you will end up with
an unknown rate. 

time x+  0:  last time polling the old device
time x+300:  first time polling the new device
time x+600:  next time polling the new device

This may generate this spike you describe (it depends on the actual
counter values on both the old and the new device).  The second interval
will be normal again.

time x+  0:  last time polling the old device
time x+  1:  insert the unknown value
time x+300:  first time polling the new device
time x+600:  next time polling the new device

The interval from x to x+300 will be unknown.  The interval from x+300
to x+600 will be calculated correctly as both values come from the new
device.

HTH
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