[mrtg] problem updating rrds

[Glen Chamberlain]

Hello all,

I'm having an issue updating mrtg rrd files.  I have mrtg configured to write to
rrds, rrdtool is installed (I'm using it with Ganglia).  If I delete the rrds and
run mrtg by hand, the rrds get properly created with data, however, on subsequent
runs of mrtg, the rrds modified time does not change and the data is the same in
the databases.  There is nothing obvious that pops out at me as being an error
when I run mrtg in debug mode.

Has anyone seen this behaviour before whereby initially creating the rrds is OK
but the rrds are not modified on subsequent runs?  It's probably a config error
on my part but I can't seem to find it.

Thanks in advance for any help you can give.

Glen