Import process creates some files where it writes the ordered sequence of key/values required for the on-disk merge algorithm.
Those files are created using "delete on exit" option so that when the import process is terminated, those files are deleted.
The problem is that in the case of an online import, the process in charge of importing the data (the directory server) will not exit after the import and so, files will not be deleted. Those temporary files will accumulate in the server's tmp directory.
In addition of the "delete on exit", a solution would be to delete programmatically the files at the end of the import process.