I moved my Cacti install from one host to another. After dealing with some other issues the poller seemed to be running more-or-less alright. But the rrd files in the rra directory weren't updating and the graphs weren't displaying. There weren't any obvious errors in cacti.log. Looking around on the web folks mostly pointed to issues with file permissions on the rrd files or the rra directory. Sure enough, I had that problem (in the process of moving I upgraded to a new version of Cacti and switched to Dag's RPM package, which uses a 'cacti' user with a different UID than the 'cactiuser' user that my previous package had used). So I fixed that and waiting for the poller to run again, but still no updates to the rra directory. I thought I'd check that rrdtool was working, so I tried to dump out one of the rrd files:

% rrdtool dump /var/www/cacti/rra/blah.rrd
ERROR: This RRD was created on other architecture

Indeed, in this move I had gone from RHEL 4 x86 to RHEL 5 x86_64. I suppose I could have gone back to the old machine and exported the data and blah blah, but I just blew away all of the files in the rra directory and the next time the poller ran it created new rrds and all seems happy now.