We recently had an issue with our backup server which was also running Nagios including pnp4nagios to gather performance data.
We quickly started to deploy a new Nagios server which started gathering statistics again right away.
After pulling the historical RRD databases from the backup we discovered no easy way to integrate the both datasets. After fiddling with some tools we extended an existing script that can be used to integrate different RRD sources into a single file to match our use case.
The resulting script simply replaces all “null” data rows in the new database that the old database has data available for. A second script provides the ability to merge whole directory trees of RRDs.
The script can be found in the rrdmerge bitbucket repository.
gocept blog 