According to http://filext.com/file-extension/RRD
, that filename extension has at least three common uses, one of which seems to be purely Windows oriented. I guess you should see whether one of them might be relevant to your system, and then you can decide whether the files can be deleted or moved. Since two of your files seem to be at least seven years old, it seems possible that they are no longer relevant. Maybe you can offload them to a place with more space, compressed if possible. When someone cries that their application has broken, maybe your question will be answered.
Maybe you can add some space to your var partition. Or, move the db directory to a non-full partition, and make symlinks to the new location.