Messages like this one show up in the log file
23.12.2009 09:50:29 *INFO * MultiIndex: Unable to delete obsolete index: _ld (MultiIndex.java, line 1084)
They mean that the index tried to delete an index segment that is not needed anymore, but was unable to delete the segment. Usually this is caused by a query that still reads from the segment. The index will simply retry to delete the segment at a later time.
This message can safely be ignored.