condor_preen examines the directories belonging to Condor, and removes extraneous files and directories which may be left over from Condor processes which terminated abnormally either due to internal errors or a system crash. The directories checked are the LOG, EXECUTE, and SPOOL directories as defined in the Condor configuration files. condor_preen is intended to be run as user root (or user condor) periodically as a backup method to ensure reasonable file system cleanliness in the face of errors. This is done automatically by default by the condor_master. It may also be explicitly invoked on an as needed basis.
When condor_preen cleans the SPOOL directory, it always leaves behind the files specified in the VALID_SPOOL_FILES list in your config file. For the log directory, the only files removed or reported are those listed in the INVALID_LOG_FILES list. The reason for this difference is that, in general, you want to leave all files in the LOG directory alone, with a few exceptions (namely, core files). condor_preen still works if you supply a VALID_LOG_FILES list instead, but this usage is depricated. There are new log files for different things introduced all the time, and you wouldn't want to have to keep updating the list of files to leave alone in the LOG directory. For example, the SMP startd can spawn an arbitrary number of condor_starter processes, each with its own log file. On the other hand, there are only a small, fixed number of files in the SPOOL directory that the condor_schedd needs to keep around, so it is easier to specify the files you want to keep instead of the ones you want to get rid of.
See the Condor Version 6.1.2 Manual for additional notices.